Since Brexit, a British citizen living in France remains a third-country national, even with a Withdrawal Agreement residence permit in hand. The carte de séjour (residence permit) protects your right to stay, but it does not give you the right to vote in national elections, it does not give you unrestricted access to certain public posts, and it leaves you exposed each time you renew. French nationality changes that equation entirely. Once you are French, you are a European Union citizen again, with a French passport, a French national identity card, and no further permit to renew. This guide explains, for a British reader, the two routes that matter in practice after Brexit: naturalisation by decree (naturalisation par décret) after years of residence, and acquisition by declaration through marriage to a French citizen (déclaration à raison du mariage). It explains every French term at first use, sets out the residence, language and good-character conditions as they appear in the Civil Code (Code civil), answers the dual citizenship question on both the French and the British side, and shows how a refusal, an adjournment (ajournement) or a Government opposition (opposition du Gouvernement) can be challenged before the French courts. The article closes with the practical position in Paris and the Île-de-France, where most British applications in the region are examined. Readers who are still at the residence-permit stage may find our guide to long-stay visas and cartes de séjour for British newcomers useful background, and spouses will want to read our piece on marrying a French national and securing a spouse permit alongside this one.

I. How does a British citizen become French after Brexit: naturalisation by decree or declaration by marriage?

The French Civil Code offers British applicants two quite different doors. Naturalisation by decree is a favour of the State: you ask, the administration assesses whether it is in the interest of France to make you French, and it can say no even when the boxes appear ticked. Acquisition by marriage is a declaration of right: once the statutory conditions are met, you declare yourself French, and the Government can only block you within a fixed period and on limited grounds. Choosing the wrong door, or preparing one as if it were the other, is the most common reason British files fail. The sections below set out each route exactly as the texts frame it.

A. Can a British resident be naturalised after five years of residence in France?

Naturalisation by decree is defined in one sentence of the Civil Code: “Hors le cas prévu à l’article 21-14-1 , l’acquisition de la nationalité française par décision de l’autorité publique résulte d’une naturalisation accordée par décret à la demande de l’étranger.” Three words carry the whole weight of the procedure for a British applicant. Par décret means the decision is taken by the executive, published as a decree, and remains discretionary. À la demande de l’étranger means nothing happens unless you file a formal application with a complete file. And autorité publique means the administration judges the opportunity of making you French, not merely whether your papers are in order. A British retiree in the Dordogne, a City secondee in Paris La Défense, and a young graduate in Lyon are all l’étranger in this sentence, and all three must persuade, not merely prove.

The first condition is residence. The Code states: “Sous réserve des exceptions prévues aux articles 21-18 , 21-19 et 21-20 , la naturalisation ne peut être accordée qu’à l’étranger justifiant d’une résidence habituelle en France pendant les cinq années qui précèdent le dépôt de la demande.” The five-year period (stage, the qualifying residence period) runs backwards from the date you file, not from the date the administration decides. Time spent as a student, time under a visitor permit, and short absences can complicate the count, because the administration looks for a résidence habituelle, a settled habitual residence, with the centre of your family, professional and financial interests in France. A British consultant who keeps a flat in London, pays most of his tax there, and spends three days a week in France will struggle to show that his habitual residence is French, even if he holds a valid permit. The related rule on the day of the decree is equally strict: “Nul ne peut être naturalisé s’il n’a en France sa résidence au moment de la signature du décret de naturalisation.” If you move back to the United Kingdom after filing and before the decree is signed, the file fails on this ground alone. Keep your home, your tax residence and your daily life in France until the decree is published.

The five-year period can be shortened in three cases that matter to British applicants. The Code provides: “Le stage mentionné à l’article 21-17 est réduit à deux ans : 1° Pour l’étranger qui a accompli avec succès deux années d’études supérieures en vue d’acquérir un diplôme délivré par une université ou un établissement d’enseignement supérieur français ; 2° Pour celui qui a rendu ou qui peut rendre par ses capacités et ses talents des services importants à la France ; 3° Pour l’étranger qui présente un parcours exceptionnel d’intégration, apprécié au regard des activités menées ou des actions accomplies dans les domaines civique, scientifique, économique, culturel ou sportif.” A British student who completes a two-year French master’s degree, a researcher recruited by a French laboratory, or a founder whose company creates significant employment in France may therefore apply after two years rather than five. The reduction is not automatic; you must claim it and document it. Your degree certificates, employment contracts, publications, or evidence of civic contribution go in the file from the start, because the prefecture (préfecture, the local State authority that examines the file) will not guess your case for you.

The second condition is assimilation (assimilation à la communauté française), meaning integration into the French community. The central text reads: “Nul ne peut être naturalisé s’il ne justifie de son assimilation à la communauté française, notamment par une connaissance suffisante, selon sa condition, de la langue, de l’histoire, de la culture et de la société françaises, dont le niveau et les modalités d’évaluation sont fixés par décret en Conseil d’Etat, et des droits et devoirs conférés par la nationalité française ainsi que par l’adhésion aux principes et aux valeurs essentiels de la République.” The same article continues with the language standard itself: “L’intéressé justifie d’un niveau de langue lui permettant au moins de comprendre le contenu essentiel de sujets concrets ou abstraits dans un texte complexe, de communiquer avec spontanéité, de s’exprimer de façon claire et détaillée sur une grande variété de sujets.” In practice this is an upper-intermediate to advanced oral and written standard, tested at the prefecture interview (entretien d’assimilation, the integration interview) and documented by an approved language certificate or diploma. A British applicant who speaks fluent holiday French but cannot discuss a newspaper article, explain the organisation of the Republic, or describe the rights and duties of a citizen will be found wanting. The interview also covers history, culture and society: the values of liberty, equality and fraternity, the principle of secularism (laïcité, the French separation of religion and the State), the organisation of elections, and everyday civic life. The file ends with a formal step the Code describes plainly: after the assimilation check, the applicant signs the Charter of the rights and duties of the French citizen (charte des droits et devoirs du citoyen français), which restates the principles, values and symbols of the Republic. Treat that signature as the seal on everything you have said at interview, and prepare accordingly.

The third condition is insertion, meaning stable professional and financial integration. The Code does not fix a salary threshold, which surprises many British applicants used to the points-based logic of the UK system. Instead, the minister examines your whole career since arrival: continuity of employment, stability of resources, payment of tax, and autonomy from welfare. The leading recent illustration is a decision of the Nantes administrative court of appeal concerning a Senegalese worker, whose reasoning applies equally to a British file. The court records that “le ministre de l’intérieur a décidé, le 10 septembre 2020, d’ajourner à deux ans sa demande au motif que l’examen de son parcours professionnel, apprécié dans sa globalité depuis son entrée en France, ne permet pas de considérer qu’il a pleinement réalisé son insertion professionnelle puisqu’il ne dispose pas de ressources stables.” The legal basis for that adjournment is the provision the court quotes in the same judgment: where the minister considers that naturalisation should not be granted, he rejects the application, and “Il peut également en prononcer l’ajournement en imposant un délai ou des conditions”, that is, he may instead adjourn the application and impose a waiting period or conditions. For a British applicant this means that a patchwork of short fixed-term contracts (contrats à durée déterminée), long gaps, undeclared freelance income, or dependence on benefits can lead to a two-year adjournment even when residence and language are satisfactory. Salaried British applicants should file continuous payslips, employment certificates, tax notices (avis d’imposition, the annual income-tax assessment), and proof of stable housing. Self-employed applicants should file balance sheets, tax returns, and evidence that the business genuinely operates from France.

The fourth condition is good character, examined through the criminal record. The Code bars acquisition outright in defined cases: “Nul ne peut acquérir la nationalité française ou être réintégré dans cette nationalité s’il a été l’objet soit d’une condamnation pour crimes ou délits constituant une atteinte aux intérêts fondamentaux de la Nation ou un acte de terrorisme, soit, quelle que soit l’infraction considérée, s’il a été condamné à une peine égale ou supérieure à six mois d’emprisonnement, non assortie d’une mesure de sursis.” The same article extends the bar to persons under an unlifted expulsion order, an unexecuted ban from French territory, or in irregular stay. Spent convictions that have been removed from the second part of the criminal record (bulletin n° 2 du casier judiciaire) through ordinary or judicial rehabilitation are excluded from this bar, but any recent custodial sentence of six months or more without suspension, any terrorism or State-security conviction, and any irregular immigration status at the date of application will stop the file. British applicants with a historic drink-driving endorsement or a spent caution should check exactly what appears on bulletin n° 2 before filing, and take advice on whether rehabilitation has operated, rather than hoping the prefecture will overlook it.

B. Can a British spouse of a French citizen apply after four years of marriage?

The marriage route is structurally more favourable than naturalisation, and many British husbands, wives and civil partners of French citizens overlook it. The governing sentence reads: “L’étranger ou apatride qui contracte mariage avec un conjoint de nationalité française peut, après un délai de quatre ans à compter du mariage, acquérir la nationalité française par déclaration à condition qu’à la date de cette déclaration la communauté de vie tant affective que matérielle n’ait pas cessé entre les époux depuis le mariage et que le conjoint français ait conservé sa nationalité.” Four elements deserve close attention. First, the four-year clock runs from the wedding, not from arrival in France. A British woman who married a Frenchman in London in 2021 and moved to France in 2023 can declare in 2025. Second, the communauté de vie, the shared married life both emotional and material, must have been continuous from the wedding to the declaration. Separation, even amicable and temporary, breaks the chain. Third, the French spouse must still be French on the day of the declaration. Fourth, where the marriage was celebrated abroad, it must first have been transcribed onto the French civil-status registers (transcription sur les registres de l’état civil français). A British-French couple married at a London register office must therefore have the marriage entered on the Nantes registers before the declaration is receivable.

The four-year period becomes five years in two situations the same article defines, and British couples frequently fall into one of them without realising it. The period is extended where the foreign spouse cannot show three years of uninterrupted lawful residence in France since the wedding, or cannot show that the French spouse was registered on the register of French citizens living abroad (registre des Français établis hors de France) throughout their shared life overseas. In plain terms: a British husband who married a French wife in Manchester, lived with her in the United Kingdom for four years, and never saw her register at the French consulate, must wait five years, not four. A British wife who joined her French husband in France one year after the wedding and has only two years of French residence must also wait five years. Check the consular registration history and the residence history before choosing your filing date, because filing a month too early renders the whole declaration inadmissible.

Declarations are received by the administration, and the receipt matters enormously. The Code states: “Les déclarations de nationalité souscrites en raison soit du mariage avec un conjoint français, en application de l’article 21-2 , soit de la qualité d’ascendant de Français, en application de l’article 21-13-1 , soit de la qualité de frère ou sœur de Français, en application de l’article 21-13-2, sont reçues par l’autorité administrative.” The same article adds that a receipt (récépissé) is issued once the documents proving admissibility have been lodged. That receipt starts the Government’s two-year opposition window. The opposition power itself is framed restrictively: “Le Gouvernement peut s’opposer par décret en Conseil d’Etat, pour indignité ou défaut d’assimilation, autre que linguistique, à l’acquisition de la nationalité française par le conjoint étranger dans un délai de deux ans à compter de la date du récépissé prévu au deuxième alinéa de l’article 26” The phrase autre que linguistique is crucial: on the marriage route, unlike naturalisation, the Government cannot oppose you for poor French. It can oppose you for unworthiness (indignité) or for non-linguistic lack of assimilation, within two years of the receipt, by decree in the Council of State (décret en Conseil d’Etat). If no opposition comes within that period, you are French. The spouse’s French is still examined at the prefecture interview, and a declaration can be refused registration for language reasons at the outset, but once the receipt is issued the linguistic ground drops away from the opposition stage.

Two recent Council of State (Conseil d’Etat, the supreme administrative court) decisions show how this opposition works in opposite directions, and both repay careful reading by British spouses. In a judgment of 29 May 2026 annulling an opposition decree, the court held: “Il ne ressort pas des éléments versés au dossier par le ministre de l’intérieur, en particulier du rapport de synthèse du 26 septembre 2022 des services de la préfecture de l’Oise chargés de l’instruction du dossier et du compte-rendu d’entretien complémentaire du 13 mai 2024, que Mme A… adopte un comportement ou soutient des thèses manifestant un rejet des principes essentiels de la République ou se tient délibérément à l’écart de la communauté nationale.” The administration had the prefecture report and the follow-up interview record, and neither showed rejection of republican principles or deliberate withdrawal from national life, so the opposition was unlawful. Contrast the judgment of 26 November 2024 upholding an opposition, where the court found: “il ressort toutefois de l’ensemble des pièces du dossier que Mme A… a organisé sa vie dans le cadre exclusif de sa communauté, en dehors de toute volonté de mixité sociale en choisissant d’adopter un mode de vie incompatible avec les valeurs essentielles de la société française et notamment avec le principe d’égalité des sexes.” Living exclusively within one community, without any openness to wider social life, and adopting a way of life incompatible with equality between the sexes, justified the finding of non-assimilation. For a British spouse, the lesson is concrete: joint life with the French partner, shared friendships, children’s schooling, participation in local life, and respect for equality in daily conduct are the evidence that defeats an opposition. A file that shows two parallel lives under one roof invites one.

British spouses should also prepare the language and community-of-life evidence from day one. Although linguistic failure alone cannot support an opposition once the receipt exists, the prefecture interview still tests spoken French, knowledge of French institutions, and the reality of married life: how you met, where you live, how household expenses are shared, whether the French spouse’s family knows you, and how the children, if any, are raised. Utility bills in both names, a joint tax return (déclaration commune), a joint bank account, photographs of family life, and consistent statements at interview all corroborate the communauté de vie. Where the couple married abroad, file the transcription certificate early, because the prefecture will not issue the receipt without it. And where the French spouse lived in Britain, retrieve the consular registration certificate now: it is the document that keeps you in the four-year track rather than the five-year one.

II. What happens to your British passport and how do you challenge a refusal in France?

Becoming French does not mean ceasing to be British, and being refused does not mean the end of the road. This second part answers the two questions every British applicant asks once the file is lodged: do I keep my United Kingdom passport, and what do I do if the administration says no. It also gives the Paris and Île-de-France practical points, because the prefectural platform that handles the region applies national law with its own timetables and habits.

A. Do you keep your British citizenship and what does French nationality change for your family?

France accepts dual nationality (double nationalité, holding two citizenships at once) as a general principle, and the United Kingdom does the same. A British citizen who becomes French by decree or by marriage therefore keeps the British passport unless British law itself required renunciation for the specific route, which it does not for these French procedures. The United Kingdom government’s guidance on dual citizenship confirms that British citizens may hold another nationality alongside their own, and French law contains no requirement for a British adult to renounce British citizenship in order to be naturalised or to declare nationality by marriage. You will travel with two passports, enter the United Kingdom as a British citizen, enter France as a French citizen, and vote in both countries where the franchise allows. The practical consequence most British families feel first is European: as a French citizen you are a Union citizen again, free to live, work and study anywhere in the European Union and the European Economic Area without a permit, and free to re-enter France after any length of absence. Practical guidance on the British side is set out on the United Kingdom government’s dual citizenship pages, and the French procedure, conditions and effects are described on the service-public.fr naturalisation pages, which every applicant should read alongside the Code.

Children need separate attention, because British parents in France often assume that a parent’s naturalisation automatically makes the children French. It does not operate that way for every child. Minor children of a naturalised parent may benefit from the collective effect (effet collectif) of the decree where they live with the parent and the decree names them, but each child’s residence and age conditions must be checked individually. Children born in France to British parents have their own declaration route. The Code provides: “L’enfant mineur né en France de parents étrangers peut à partir de l’âge de seize ans réclamer la nationalité française par déclaration, dans les conditions prévues aux articles 26 et suivants si, au moment de sa déclaration, il a en France sa résidence et s’il a eu sa résidence habituelle en France pendant une période continue ou discontinue d’au moins cinq ans, depuis l’âge de onze ans.” A British teenager born in Paris, schooled in France since the age of eight, can therefore claim French nationality from sixteen, and parents can claim on the child’s behalf from thirteen where continuous residence since the age of eight is shown. File the child’s claim separately, with school certificates (certificats de scolarité) covering every year relied upon, rather than assuming the parent’s decree sweeps the child along.

For readers in Paris and the Île-de-France, three local points strengthen the application. First, the examination platform for the region centralises a very large number of files, and interview waiting times are longer than in smaller departments. File early in the residence window, answer every request for additional documents within the stated time, and keep copies of everything transmitted through the online filing system. Second, housing evidence in Paris carries particular weight because short-term furnished lets, parental hospitality, and frequent moves can blur the picture of résidence habituelle. A continuous lease or title deed, electricity and insurance bills in your name, and a stable tax address do more for the file than any covering letter. Third, the language interview in the region is conducted to the national standard without leniency for the capital’s international atmosphere. Living in an English-speaking bubble in the 8th arrondissement, working entirely in English, and socialising only with other Britons is the fastest route to an assimilation finding against you. Enrol in a French course, join a local association, and make sure the file shows it.

Readers who also hold property, receive United Kingdom rental income, or draw a British pension should keep the nationality project separate from the tax position in their minds, while preparing both coherently. Nationality does not itself change the France–United Kingdom double tax treaty, and becoming French does not exempt a British-source pension from the treaty’s allocation rules. Our guides to dual tax residence and the treaty tie-breaker and to British landlords taxed on English lets while living in France explain the parallel tax analysis. The nationality file should still be consistent with the tax file: a declared French tax residence, French tax returns, and French social-security affiliation all corroborate habitual residence and insertion, while a file that proclaims French integration alongside a declared non-resident tax status invites scepticism.

B. How do you challenge a refusal, an adjournment or a Government opposition?

French nationality litigation has its own map, and using the wrong court or missing the time limit ends the case before it begins. Start with naturalisation. Where the minister considers that the application should not succeed, he issues a refusal (rejet); where the file is promising but premature or incomplete in insertion terms, he issues an adjournment imposing a waiting period or conditions, as the Nantes case above illustrates with its two-year postponement. Both decisions are administrative acts that can be challenged first by an informal appeal to the administration (recours gracieux, a request to reconsider addressed to the minister, or recours hiérarchique to the superior authority) and then by judicial review before the administrative court. The court with territorial jurisdiction for refusals and adjournments of naturalisation is the Nantes administrative court (tribunal administratif de Nantes), whose judgments in this field are legion, including the 21 June 2024 judgment annulling a two-year adjournment that the minister then appealed to the Nantes court of appeal in the 23 May 2025 case cited above. The time limit for that judicial challenge follows the general rule of administrative litigation: “La juridiction ne peut être saisie que par voie de recours formé contre une décision, et ce, dans les deux mois à partir de la notification ou de la publication de la décision attaquée.” Diarise two months from receipt of the refusal or adjournment, allow for the effect of any timely informal appeal on that period, and file a complete pleading with the evidence the administration overlooked, not a bare protest letter.

On the marriage route the litigation map splits in two. Refusal to register the declaration (refus d’enregistrement) goes to the judicial courts (ordre judiciaire), because registration concerns civil status. Government opposition by decree, once the receipt has been issued, goes to the Council of State by way of an action for excess of power (recours pour excès de pouvoir, the claim asking the administrative supreme court to annul an unlawful administrative act), as both the 29 May 2026 annulment and the 26 November 2024 rejection show: each applicant asked the Council of State to annul the decree opposing acquisition, one successfully where the file showed no rejection of republican principles, the other unsuccessfully where the file showed a closed, separate life. The same two-month logic governs the challenge to the opposition decree, running from its notification, and the pleading must meet the administration’s assimilation case document by document: prefecture synthesis report, interview record, evidence of shared life, language proof, and testimony of integration. Attack the exact ground the decree states. If the decree alleges lack of assimilation through community withdrawal, answer with proof of mixed social life, schooling, associations, and daily respect for equality. If it alleges unworthiness through criminal history, answer with the precise scope of “Nul ne peut acquérir la nationalité française ou être réintégré dans cette nationalité s’il a été l’objet soit d’une condamnation pour crimes ou délits constituant une atteinte aux intérêts fondamentaux de la Nation ou un acte de terrorisme, soit, quelle que soit l’infraction considérée, s’il a été condamné à une peine égale ou supérieure à six mois d’emprisonnement, non assortie d’une mesure de sursis.” and show rehabilitation, removal from bulletin n° 2, or the inapplicability of the cited conviction. General character references do not answer a specific assimilation finding.

Evidence wins these cases, and the evidence that matters is contemporaneous. Keep the prefecture interview convocation, prepare a written chronology of residence with a document for every address, and bring originals plus copies to the interview. For assimilation, file the language diploma, course enrolments, payslips, tax notices, school certificates for the children, association memberships, and any civic training certificates. For the marriage route, add the wedding certificate, the transcription certificate for overseas marriages, the French spouse’s proof of nationality and consular registration history, joint tax returns, joint bank statements, the lease or title in both names, and proof that the communauté de vie never ceased. Where the administration relies on a prefecture report you have not seen, request communication of the file, compare the report’s summary with the interview record, and plead any material inaccuracy precisely: wrong dates, unattributed statements, or conclusions the documents do not support. The 29 May 2026 annulment turned on exactly that comparison between the administration’s claims and what the prefecture papers actually showed. Finally, remember the reduced two-year residence track where it applies: a British graduate of a French university or a person of exceptional integration should plead “Le stage mentionné à l’article 21-17 est réduit à deux ans : 1° Pour l’étranger qui a accompli avec succès deux années d’études supérieures en vue d’acquérir un diplôme délivré par une université ou un établissement d’enseignement supérieur français ; 2° Pour celui qui a rendu ou qui peut rendre par ses capacités et ses talents des services importants à la France ; 3° Pour l’étranger qui présente un parcours exceptionnel d’intégration, apprécié au regard des activités menées ou des actions accomplies dans les domaines civique, scientifique, économique, culturel ou sportif.” from the outset, with diplomas and proof attached, rather than discovering it after a refusal based on an incomplete five-year count.

Conclusion

For a British citizen settled in France after Brexit, French nationality is the durable answer to the precariousness of permit renewals. Naturalisation by decree rewards five years of genuine habitual residence, proven assimilation to the French community with advanced French, stable professional insertion, and a clean record, with a two-year track for graduates, talents and exceptional integration. Declaration by marriage rewards four years of continuous shared life with a French spouse, or five where French residence or consular registration is missing, with transcription of overseas weddings and a receipt that starts a two-year opposition window in which only unworthiness or non-linguistic non-assimilation can block you. Dual Franco-British citizenship follows either route as a matter of course, and children born in France have their own declaration path from thirteen or sixteen. Where the administration refuses, adjourns or opposes, the remedy is a documented, time-limited challenge: informal reconsideration promptly, then the Nantes administrative court for naturalisation decisions within two months of notification, or the Council of State for opposition decrees, meeting each stated ground with contemporaneous proof. Prepare residence, language, insertion and shared life as if every page will one day be read by a judge, because in a contested file, it will be.