You are British, you have lived in France for years, and since Brexit your life here depends on a residence card that must be renewed, justified and sometimes defended. Naturalisation changes that equation permanently: a British citizen who becomes French recovers the unconditional right to live, work, vote and move within the European Union, while keeping the British passport, because both France and the United Kingdom allow dual nationality. Two main doors lead there. The first is naturalisation by decree (naturalisation par décret), a favour of the State granted after at least five years of habitual residence, an assessment interview testing your French and your assimilation, and an administrative enquiry into your conduct, your taxes and your integration. The second is nationality by marriage (acquisition par mariage), a declaration-based route open after four years of marriage to a French spouse, provided the shared life has never ceased. Both routes refuse many applicants: insufficient French, unstable resources, criminal convictions, or a file that simply fails to prove what it claims. And every refusal, adjournment or opposition can be challenged, before the administration itself or before the courts, within strict time limits. This guide explains, in order, which route fits your situation as a British citizen, the exact statutory conditions with their official texts, the procedure step by step including the Paris and Île-de-France specifics, and how to fight a refusal, using a January 2023 decision of the Paris Court of Appeal that holds a hard lesson about the language test for older applicants.
I. How does a British citizen become French after Brexit: naturalisation by decree or nationality by marriage?
A. Who gets naturalised by decree, and what do five years, assimilation and good conduct really mean?
Start with the principle many British applicants misunderstand: naturalisation by decree is not a right that follows automatically from years spent in France. It is a decision of the public authority, granted on request. Article 21-15 of the Civil Code states it word for word: “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.” In plain English: apart from the special reinstatement case, acquiring French nationality by decision of the public authority results from a naturalisation granted by decree at the foreigner’s request. Because it is a grant, not an entitlement, the administration enjoys a wide discretion, and the administrative courts review refusals only for legal error, factual error or manifest error of assessment, not for sympathy. That single idea organises everything that follows: your file must persuade, because no judge will substitute their own view of whether you deserve to be French.
The first hard condition is residence. Article 21-17 of the Civil Code provides: “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.” Five years of habitual residence in France before the application, subject only to the statutory exceptions, such as the reduced two-year period for applicants who have completed two years of higher education in France or made exceptional contributions, and the total exemption for refugees and certain other protected categories. For a British reader this raises the Brexit-specific question: do years spent in France before 2021 count? Yes. Lawful residence under European Union free movement before the end of the transition period counts toward the five years exactly like residence under a Withdrawal Agreement card afterwards, provided you can document it with tax notices, pay slips, health cover and continuous address evidence. The administration counts presence proved by papers, not presence remembered fondly. And residence must still exist at the final moment: Article 21-16 of the Civil Code adds: “Nul ne peut être naturalisé s’il n’a en France sa résidence au moment de la signature du décret de naturalisation.” No one may be naturalised unless resident in France when the decree is signed. A British applicant who moves back to Kent while the file is pending destroys the file.
The second condition is assimilation (assimilation à la communauté française), tested mainly through language, history, culture and values. Article 21-24 of the Civil Code states the full rule: “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. 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. A l’issue du contrôle de son assimilation, l’intéressé signe la charte des droits et devoirs du citoyen français. Cette charte, approuvée par décret en Conseil d’Etat, rappelle les principes, valeurs et symboles essentiels de la République française.” In practice this means level B1 spoken French of the Common European Framework, proved by an approved diploma or language certificate, or assessed during the individual assimilation interview (entretien d’assimilation) conducted by a prefecture agent. The interview also probes knowledge of French history, institutions and the rights and duties of citizenship, and it ends with the signing of the charter of rights and duties. British applicants who sailed through decades of life in France speaking English at home and school-gate French in the playground fail here more often than anywhere else, because the test measures active, precise oral expression, not survival comprehension.
The third condition is conduct and insertion. Article 21-23 of the Civil Code bars naturalisation bluntly: “Nul ne peut être naturalisé s’il n’est pas de bonnes vie et moeurs ou s’il a fait l’objet de l’une des condamnations visées à l’article 21-27 du présent code.” No one may be naturalised who is not of good life and morals or who has incurred one of the convictions listed in Article 21-27. And Article 21-27 of the Civil Code details the absolute bars: “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.” Any prison sentence of six months or more without suspension, for any offence, blocks acquisition entirely, alongside terrorism and national-security convictions, unlifted expulsion orders, unexecuted bans from French territory and irregular stay. Below that threshold, the administration still weighs everything: unpaid taxes, benefit fraud, driving disqualifications, domestic incidents recorded by the police, and professional instability. Insertion means stable, sufficient and lawful resources, generally assessed over the three years before the application, plus payment of French tax on worldwide income where France is your tax residence. A British early retiree living on a UK pension declared in France can satisfy this perfectly; a British consultant paid offshore into a London account while resident in Lyon invites refusal. The official procedure is published on the administration’s guidance page for naturalisation by decree, and the residence position that underpins the whole file is the one described in our guide to the Withdrawal Agreement residence card for British citizens.
B. How does a British spouse of a French national claim nationality by marriage, and what can still go wrong?
The marriage route is structurally kinder to the applicant, because it works by declaration (déclaration de nationalité), not by discretionary grant. Article 21-2 of the Civil Code provides: “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.” A foreigner who marries a French national may, after four years from the marriage, acquire French nationality by declaration, on condition that at the date of the declaration the shared life, both emotional and material, has not ceased since the marriage and that the French spouse has kept French nationality. Four years of marriage, a shared life that has never broken, a spouse still French: meet those three tests and the declaration must be registered, unless the State proves a legal ground to refuse registration or opposes the acquisition. The declaration is received by the administration, and Article 26 of the Civil Code organises which authority receives each category of nationality declaration, with marriage declarations going to the administrative authority. The practical steps are set out on the official page for French nationality by marriage.
Three traps catch British spouses. The first is the extended five-year period: where the couple has not resided together in France for at least three years since the marriage, or where the British declarant has not been continuously resident in France, the community-of-life period rises from four to five years, a detail that surprises couples who married in London, lived in Manchester, and moved to the Dordogne only recently. The second is interruption of shared life: any separation, even without divorce, breaks the continuity the statute demands, and the administration investigates through joint tax returns, joint leases, shared bank accounts and the prefecture interview. The third, and the most underestimated by native English speakers, is the language test, which applies to declarations by marriage exactly as it does to naturalisation. The Paris Court of Appeal gave this lesson in a decision every British spouse should read before filing. On 31 January 2023 (Pôle 3, Chamber 5, RG 21/12332), the court confirmed the rejection of a marriage-based declaration where the Minister of the Interior had refused registration because the declarant, in the court’s words, the declarant had failed to show sufficient spoken French, the prefecture officer having assessed his level below oral B1 of the Common European Framework set by Article 14 of the decree of 30 December 1993. The declarant argued that at 77 years old he should have been exempted from any language examination. The court answered with the distinction that now governs every older applicant’s file: the over-sixties exemption from producing a language diploma or certificate is not an exemption from knowing French itself, because applicants aged sixty or over still sit the individual interview that checks their level. The judgment was confirmed in full, with the Article 28 mention ordered. For British spouses in their sixties and seventies who assumed age alone would carry them through, the message is blunt: no certificate is required, but real spoken French is still verified face to face, and failure at that interview sinks the declaration.
Beyond registration, the State keeps a weapon of last resort. Article 21-4 of the Civil Code provides: “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 ou, si l’enregistrement a été refusé, à compter du jour où la décision judiciaire admettant la régularité de la déclaration est passée en force de chose jugée.” Even after a declaration is registered, the Government may oppose the acquisition by decree in the Council of State, for unworthiness or lack of assimilation other than linguistic, within two years of the receipt. Sham marriage, polygamy in fact, serious misconduct or a concealed criminal record are the classic grounds. Opposition is rarer than refusal, but its two-year shadow means a British spouse should keep conduct and papers clean well after the ceremony at the prefecture. One reassurance closes this part: becoming French does not cost you Britain. France imposes no renunciation of prior nationality in either route, and the United Kingdom expressly permits its citizens to hold another citizenship, as confirmed on the official dual citizenship guidance on gov.uk. Your children born after your acquisition may themselves be French by filiation, while children born before it keep their own position, a point to verify child by child rather than assume. For spouses whose residence position is fragile, our guide to the residence card for British spouses of French nationals explains the private-and-family-life route that protects you while the nationality file matures.
II. What happens after you apply, and how do you challenge a refusal, adjournment or opposition?
A. Where do you file, what does the enquiry examine, and what are the Paris and Île-de-France specifics?
Naturalisation applications are now filed online through the ANEF portal (Administration Numérique pour les Étrangers en France), the single digital desk for foreigners’ procedures, with the file allocated to the prefecture of your home département (administrative area). For Paris that means the Paris prefecture; for the inner ring, the prefectures of Hauts-de-Seine, Seine-Saint-Denis and Val-de-Marne; for the outer ring, Seine-et-Marne, Yvelines, Essonne and Val-d’Oise. The platform logic matters because an incomplete upload stalls silently: payslips, tax assessments (avis d’imposition), employment contracts, proof of health cover, criminal-record extracts, civil-status documents with apostille and sworn translation where needed, and the language diploma or certificate. British documents need particular care since Brexit: birth and marriage certificates should carry the apostille (apostille) under the Hague Convention, and any document in English must be translated by a sworn translator (traducteur assermenté) listed with a French court of appeal. A file with an untranslated HMRC letter or a missing apostille is not examined on the merits; it is returned or shelved.
Once complete, the file triggers the assimilation interview at the prefecture or sub-prefecture, then a full administrative enquiry: police and gendarmerie checks, tax-office consultation, verification of social-security contributions and benefits, and sometimes an employer contact or home visit. The prefect issues a reasoned opinion and forwards the file to the Ministry of the Interior, which decides. Three outcomes exist. Admission: your name appears in a naturalisation decree published in the Journal officiel (the official gazette), and you become French on the decree’s date, provided you still reside in France that day, as Article 21-16 requires. Adjournment (ajournement): the decision postpones examination, typically for two or three years, often for insufficient length of insertion, recent arrival of the family, or a language level judged improvable; it is not a refusal, but it freezes the file and restarts the clock on the missing element. Refusal (rejet): the decision closes the file on grounds such as lack of assimilation, unstable or unlawful resources, or conduct. Both adjournment and refusal must be reasoned in fact and in law, and both must state the remedies and time limits. For declarations by marriage, the mirror procedure is registration or refusal of registration by the minister, followed by the two-year opposition window described above. In Paris and the inner suburbs, waiting times between the online filing and the interview commonly stretch beyond a year, which makes early filing strategic: the five-year residence count runs to the application date, but the interview tests the French you speak on the day, so file once eligible and keep improving the language while the queue moves.
Dual nationality deserves a precise word here because British applicants often receive wrong advice from both sides of the Channel. France has never required a naturalised Briton to renounce British citizenship, and the United Kingdom has never deprived a citizen of British nationality for acquiring French nationality voluntarily. The two positions coexist without any declaration, oath of renunciation or permission to seek. The practical consequences are administrative rather than legal: two passports to renew on different cycles, entry to the United Kingdom as a British citizen and to France as a French citizen, tax residence decided by facts and the France-United Kingdom double tax treaty rather than by the passports you hold, and consular protection from either State when travelling outside both. None of this affects the nationality decision itself, but the enquiry may ask about your ties to each country, and a file showing exclusive professional, fiscal and family anchoring in Britain with only a holiday address in France will struggle on assimilation whatever the passports say.
B. How do you fight a refusal or adjournment of naturalisation, a refused marriage declaration, or a State opposition?
Every negative decision opens a two-track system: an administrative appeal that asks the administration to think again, and a court action that asks a judge to annul. For naturalisation refusals and adjournments, the first step is the optional but strategically vital administrative appeal (recours gracieux before the minister, or recours hiérarchique), filed within two months of notification, attaching every missing or improved piece of evidence: a new B1 or B2 certificate, a permanent contract replacing fixed-term work, cleared tax arrears with proof of payment, or a corrected criminal-record extract. A well-built administrative appeal succeeds precisely where the refusal rested on a snapshot that has since changed, because the minister re-examines the file as it stands at the new decision date. Silence for four months equals an implied refusal, which itself can be taken to court. The court action goes to the administrative tribunal (tribunal administratif) of your home, in Paris the Montreuil tribunal for Seine-Saint-Denis residents and the Paris tribunal for the capital proper, by an action for excess of power (recours pour excès de pouvoir) seeking annulment. Article R421-1 of the Code of Administrative Justice sets the clock: “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.” Two months from notification to seize the court, a deadline that is strict and almost never extended. Before the tribunal, remember the standard of review: naturalisation is discretionary, so the judge checks competence, procedure, accuracy of the facts, legal characterisation and manifest error of assessment, but does not re-mark your assimilation interview. The winning angles are therefore factual correction, where the refusal states something provably wrong about your taxes, work or family presence, legal error, where the administration applied a condition the statute does not contain, and disproportionality in the rare cases where the facts found cannot reasonably support the conclusion. Annulment sends the file back for re-examination; it does not itself make you French, and the administration may refuse again on a corrected ground, which is why the administrative appeal and the court action are usually run in sequence rather than as alternatives.
For declarations by marriage, the forum is different and more favourable. A refusal to register the declaration is contested before the civil courts, not the administrative courts: the declarant summons the public prosecutor (procureur de la République) before the judicial tribunal (tribunal judiciaire), asking the court to declare that the conditions are met and that the declarant is French. Here the judge decides the merits fully, including hearing the declarant’s French where language is the issue, as the Paris case shows the courts doing in detail. The January 2023 Paris ruling is the worked example to study: the court examined the age-based diploma exemption claim against the decree’s wording, adopted the first judges’ exact and relevant reasons, verified that an individual interview had indeed tested the language, and confirmed the refusal because no evidence of sufficient oral French had ever been produced. A British spouse challenging a language-based refusal must therefore arrive with the opposite file: a recent approved certificate at B1 or above, or concrete proof of daily fluent use, plus evidence rebutting each factual finding in the refusal. Where the refusal rests instead on disputed separation or alleged sham, the weapons are joint tax assessments for every year of marriage, shared leases and utility bills, children’s school records at the common address, and witness evidence of genuine shared life. And where the Government opposes an already registered declaration within its two-year window, the opposition decree itself is challenged before the Council of State (Conseil d’État), the supreme administrative court, within two months, with arguments centred on unworthiness and assimilation findings, a specialised litigation where representation by a Council of State barrister (avocat aux Conseils) is required.
Four practical rules decide most cases before they reach a judge. First, calendar everything from the notification date stamped on the decision letter, not from when you opened it, and file the administrative appeal by registered letter with acknowledgement of receipt (lettre recommandée avec accusé de réception) so the date is provable. Second, never refile an identical application after a refusal without changing the facts: a second file with the same language level and the same pay slips produces a second refusal and a reputation for persistence without progress, whereas a refile after genuine change, new certificate, stable job, regularised taxes, is a new file judged on its own. Third, keep your residence card flawless throughout: an expired card, an undeclared move or a missed renewal while the nationality file is pending hands the administration an irregular-stay ground that Article 21-27 makes absolute. Fourth, for couples, protect the marriage evidence continuously from declaration to the end of the two-year opposition period, because shared life is assessed at the declaration date and unworthiness can be raised for two years after. The British applicants who win are rarely those with the most moving story; they are those whose papers prove, line by line, residence, language, work, taxes and shared life, and who answer each stated ground of refusal with a dated document rather than an explanation.
Conclusion
Becoming French after Brexit is a realistic project for a British citizen who plans it as a legal file, not as a reward for years affectionately spent in France. Naturalisation by decree demands five documented years, residence maintained until the decree is signed, B1 spoken French verified face to face, stable lawful resources and clean conduct, all assessed through a discretionary grant that courts review only for error. Nationality by marriage offers a declaration-based route after four years of unbroken shared life with a French spouse, but it tests language just as strictly, as the Paris Court of Appeal confirmed in January 2023 when it upheld the rejection of a 77-year-old declarant whose exemption from diplomas was not an exemption from French itself, and it leaves a two-year opposition window open after registration. Both routes preserve your British citizenship, require translated and apostilled documents, run through prefectures whose Paris-region queues reward early and complete filing, and answer every refusal with a two-month appeal clock. Assemble the evidence before you apply, sit the language test only when you can genuinely pass it, keep taxes and residence cards immaculate, and challenge each negative decision on its stated grounds with dated proof. French nationality is granted to the file that proves it deserves the decree.