Since the end of the Brexit transition period, British citizens living in France are third-country nationals. A Withdrawal Agreement residence permit, the carte de séjour Accord de retrait du Royaume-Uni de l’Union européenne, or a standard long-stay visa followed by a carte de séjour secures the right to stay, work and use public services. Yet for the growing number of Britons who have made France their permanent home, residence is only half the question. The other half is nationality: how does a British adult living in Paris, Lyon, Bordeaux or a Dordogne village actually become French, what can go wrong, and what can be done when the administration says no? This guide answers those three questions in full. It explains the two legal doors that matter in practice, naturalisation by decree (naturalisation par décret) and acquisition by declaration through marriage (déclaration de nationalité par mariage), the residence, language and good-character conditions attached to each, and the precise routes for challenging a refusal, an adjournment or an endless silence. Every decisive rule is quoted from the statute or the decided case it comes from, so that a British reader can check each statement against the official French source it relies on.

I. The two doors to French nationality for a British resident after Brexit

A. Naturalisation by decree: five years’ residence, assimilation and good character for British applicants

Naturalisation by decree is the general route. It is a favour granted by the French State, not a right acquired automatically by the passage of time, and it is pronounced by decree (décret), the formal legal instrument signed at the highest level of the executive. The Civil Code (Code civil) states the principle in one sentence: “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.” Because it is a decision of the public authority, the administration enjoys a margin of appreciation, but that discretion is framed by precise statutory conditions, and a refusal that ignores them can be challenged before the administrative courts, as Section II explains.

The first condition is residence, and it has two limbs. At the moment the decree is signed, the applicant must actually live in France: “Nul ne peut être naturalisé s’il n’a en France sa résidence au moment de la signature du décret de naturalisation.” Before that, the applicant must show five years of habitual residence (résidence habituelle) in France preceding the application: “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.” For a British reader, habitual residence means the real centre of daily life: the French home, tax residence, children’s schooling and actual physical presence. Years spent in France before Brexit under EU freedom of movement count exactly like years spent since, provided the applicant held lawful residence throughout. Time as a visitor under the ninety-day Schengen allowance does not count, which is why Britons who arrived after Brexit on a visitor visa and only later obtained a carte de séjour must count their five years from stable, lawful settlement, not from their first viewing trip. Readers who are still sorting out their residence basis should start with our pillar guide to which long-stay visa and which carte de séjour a British newcomer needs, and holders of a Withdrawal Agreement card should keep our guide to renewing the WARP card and challenging a refusal alongside this article, because lawful residence is the foundation on which every nationality application rests.

The five-year period, lawyers call it the stage, can be shortened. The statute 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.” British graduates of a French university, researchers, entrepreneurs whose business genuinely contributes to the French economy, artists and sportspeople can therefore apply after two years. In rarer cases the residence condition disappears entirely. The statute lists the cases that may be naturalised with no qualifying period of residence, including service in the French forces, as “4° L’étranger qui a effectivement accompli des services militaires dans une unité de l’armée française ou qui, en temps de guerre, a contracté un engagement volontaire dans les armées françaises ou alliées ;” those who have rendered exceptional services, as “6° L’étranger qui a rendu des services exceptionnels à la France ou celui dont la naturalisation présente pour la France un intérêt exceptionnel.” and recognised refugees, as “7° L’étranger qui a obtenu le statut de réfugié en application de la loi n° 52-893 du 25 juillet 1952 portant création d’un Office français de protection des réfugiés et apatrides.”

The second condition is assimilation to the French community (assimilation à la communauté française), which covers language, history, culture and republican values. The Civil Code provides: “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.” In practice this is tested in two stages. The applicant produces a language diploma or certificate at the required level, currently level B2 of the Common European Framework of Reference for Languages for naturalisation by decree, and then sits an individual interview at the préfecture, the State’s local authority in each département, where an official verifies spoken French and asks about French history, institutions and the rights and duties of citizenship. At the end of the examination the applicant signs the Charter of the rights and duties of the French citizen (charte des droits et devoirs du citoyen français). The language paragraph of the same article sets the bar in plain terms: “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.” British applicants who use English at work and French only for shopping should take this level seriously and prepare well before filing, because an insufficient interview is one of the most common grounds for adjournment, the decision that postpones the application rather than rejecting it outright.

The third condition is good character and a clean criminal record. The statute states: “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.” The cross-referenced article 21-27 excludes, among others, anyone sentenced to six months’ imprisonment or more without suspension: “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 also bars anyone whose residence is irregular. The administration checks the second bulletin of the criminal record (casier judiciaire, bulletin n°2), and the official service-public.fr guidance confirms that spent or excluded convictions are treated differently, but the working rule for a British applicant is simple: any recent custodial sentence, any drink-driving conviction carrying imprisonment, and any period of unlawful stay must be disclosed and explained with evidence of rehabilitation, because concealment discovered later can destroy the decree itself, not just the application. Applications are now filed online through the ANEF portal (Administration numérique pour les étrangers en France), a State stamp duty (droit de timbre) is payable at the current amount shown on service-public.fr, and a receipt (récépissé) is issued once the file is complete. From that receipt, the clock starts: “La réponse de l’autorité publique à une demande d’acquisition de la nationalité française par naturalisation doit intervenir au plus tard dix-huit mois à compter de la remise de toutes les pièces nécessaires à la constitution d’un dossier complet contre laquelle un récépissé est délivré immédiatement.” Where the applicant has lived habitually in France for at least ten years on that date, the period falls to twelve months. These deadlines matter because they mark the point from which an applicant left without any answer can move to the challenge stage described in Section II.

One reassurance before turning to marriage: becoming French does not mean ceasing to be British. The United Kingdom expressly permits its citizens to hold British citizenship together with the citizenship of another country, as the official gov.uk guidance on dual citizenship confirms. France takes the same position, so a naturalised Briton keeps the British passport, continues to pass British citizenship to children born abroad within the limits of British law, and simply adds French and therefore European Union citizenship, with the right to vote in all French elections and to move freely across the Union, rights that Brexit had removed.

B. Declaration through marriage to a French spouse: four years, shared life and registration for British husbands and wives

The second door is reserved for Britons married to a French citizen. It works by declaration (déclaration de nationalité), a unilateral legal act by which the foreign spouse claims French nationality, rather than by decree. The Civil Code opens it after four years of marriage: “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.” Three cumulative requirements sit inside that single sentence, and each of them needs unpacking for a British couple.

First, the marriage itself must be valid and lasting. Four years run from the wedding day, extended to five where the foreign spouse cannot show three years of continuous lawful residence in France since the marriage or cannot prove the French spouse’s registration with the French consular register while living abroad. Same-sex marriages celebrated in the United Kingdom are fully recognised, and a marriage celebrated in Gretna Green, Gibraltar or elsewhere is effective provided it is valid under the law of the place of celebration and transcribed where transcription is required. The French spouse must still hold French nationality on the day of the declaration, so a spouse who has lost or renounced French nationality without reacquiring it closes this door until the position is regularised.

Second, the couple must still share a genuine common life, affective and material (communauté de vie tant affective que matérielle), on the very date of the declaration. This is the condition that destroys most weak files. Separate addresses, a spouse who has returned to live in the United Kingdom while the other remains in France, separate finances with no shared household, or a relationship kept alive only on paper will all be detected, because the préfecture investigates and the public prosecutor (ministère public) can oppose registration. The courts apply this test strictly and assess it at the date of the declaration, not across the whole marriage. In a judgment of 5 September 2023 the Paris Court of Appeal confirmed the rejection of a declaration where the evidence only proved family life during the years the children were born but nothing at the decisive date, holding that “la preuve de la continuité de la vie commune, tant affective que matérielle, n’étant pas rapportée à la date de la déclaration, le jugement est confirmé”, before adding “Confirme le jugement,” and ordering the statutory marginal note (mention prévue par l’article 28 du code civil) on the civil-status records. British-French couples should therefore build their file around the date of the declaration: joint tenancy agreements or title deeds, joint tax notices (avis d’imposition), joint bank statements, electricity and insurance bills in both names, school certificates showing the same address, and travel and correspondence showing a lived-in relationship. Family attestations help only if they are precise and detailed; the same Paris judgment rejected vague statements from relatives and neighbours who merely affirmed that the couple lived together without dates or circumstances.

Third, the British spouse must show sufficient knowledge of the French language, assessed at level B1 of the Common European Framework for this route. The Paris Court of Appeal recalled the rule when it confirmed a refusal to register the declaration of a 77-year-old applicant, emphasising that age alters the method of proof, not the substance of the condition. The court held that “la dispense de production d’un diplôme ou d’une attestation d’un niveau de langue égal ou supérieur à celui exigé en application de l’article 14 dont bénéficient notamment les personnes âgées d’au moins soixante ans, ne constitue pas une dispense de la connaissance de la langue française”, adding that “les déclarants âgés d’au moins 60 ans faisant l’objet selon l’article 15 du décret d’un entretien individuel destiné à vérifier leur niveau de français.” The judgment was therefore confirmed: “Confirme le jugement ;” Older British spouses, a common profile among retirees in France, should read this carefully. Being over sixty spares the applicant from producing a diploma, but not from speaking French at the individual interview. Preparation with a qualified teacher, rather than reliance on years of passive residence, is what carries the file.

Once the declaration is signed before the competent authority and the file is complete, the administration registers it (enregistrement) unless it refuses registration for failure to meet the legal conditions. Registration makes the declarant French retroactively from the date of the declaration, which is why the State keeps two powerful rights of second look. Within two years of registration, the public prosecutor may challenge it where the legal conditions were not met: “Dans le délai de deux ans suivant la date à laquelle il a été effectué, l’enregistrement peut être contesté par le ministère public si les conditions légales ne sont pas satisfaites.” In cases of lies or fraud, the two years run from discovery: “L’enregistrement peut encore être contesté par le ministère public en cas de mensonge ou de fraude dans le délai de deux ans à compter de leur découverte.” And a break-up immediately after registration creates a legal presumption against the declarant: “La cessation de la communauté de vie entre les époux dans les douze mois suivant l’enregistrement de la déclaration prévue à l’article 21-2 constitue une présomption de fraude.” A British spouse who separates or divorces within the first year therefore faces a presumption of fraud, a presumption that can be rebutted with evidence but that reverses the burden of argument. The lesson is practical as well as legal: marry, declare and live genuinely, keep every proof of shared life from the declaration date onwards, and treat the first year after registration as a period of continued vigilance rather than a finish line.

The highest civil court has also set the framework for contesting nationality obtained by marriage where private and family life is at stake. On 18 June 2025 the First Civil Chamber of the Court of Cassation (Cour de cassation, première chambre civile), appeal number 23-23.456, ruled on a challenge to the registration of a declaration made under article 21-2, the public prosecutor having acted after discovering children born outside the marriage and after the couple’s divorce. The Court recalled the European Convention test in these terms: “Il résulte de la jurisprudence de la Cour européenne des droits de l’homme que bien que le droit à la nationalité ne soit pas en tant que tel garanti par la Convention de sauvegarde des droits de l’homme et des libertés fondamentales ou par ses Protocoles, il n’est pas exclu qu’un refus arbitraire de nationalité, ou une déchéance arbitraire de la nationalité, puisse dans certaines conditions poser un problème au regard de l’article 8 de la Convention à raison de l’impact que pareille décision peut avoir sur la vie privée de la personne concernée”, and confirmed that judges must check both arbitrariness and concrete consequences. In that case the annulment stood, because the measure was provided by law, the authorities had acted diligently and promptly, the applicant had enjoyed adequate judicial review, and he showed no concrete consequence such as statelessness or removal. The Court therefore ended with “REJETTE le pourvoi ;” The message for British declarants is balanced. Fraud findings survive European human-rights review where the procedure is fair and prompt, but a genuinely arbitrary refusal, or one with devastating unexamined consequences, does not. Evidence and procedure decide everything, which leads directly to the second half of this guide: what refusal looks like, and how to fight it.

II. When France says no: refusals, delays and how a British applicant challenges them after Brexit

A. What refusal looks like for British applicants: inadmissibility, rejection, adjournment, silence and the classic traps

For naturalisation by decree, an unfavourable outcome takes three distinct legal forms, and confusing them is the first mistake. Inadmissibility (irrecevabilité) means the file fails a threshold condition, typically the completeness of the documents or the five-year residence, and the application is not even examined on the merits. Rejection (rejet) means the file was examined and found wanting on substance: insufficient assimilation, doubtful good character, or a centre of interests still abroad. Adjournment (ajournement) postpones the decision, usually for two years, inviting the applicant to reapply once a remediable weakness, most often the French level or the stability of professional integration, is cured. Each form carries different reasoning and different prospects on challenge, so the first task on receiving the decision is to identify which box it falls into and which findings of fact it rests on.

Four traps account for most British refusals. The first is the language interview. The préfecture interview is not a friendly chat; it is an examination, and nervous candidates who hold a valid certificate can still fail the oral assessment if they cannot discuss abstract subjects spontaneously. The Paris language judgment quoted above shows that even advanced age offers no shelter, and younger applicants get none at all. The remedy is preparation, not protest: structured courses, certificates from recognised bodies, and regular documented use of French before the interview date. The second trap is the criminal record and, more broadly, the assessment of good character. A British applicant with an old United Kingdom conviction must understand that French authorities can take foreign convictions into account and will in any event examine the full story told by the file. Minor road-traffic penalties and long-spent matters are viewed differently from recent custodial sentences, but only a frank file with proof of rehabilitation allows the administration to exercise its discretion favourably. The statutory wall remains article 21-27, quoted in full above: serious convictions and irregular stay exclude acquisition as of right, and concealment converts a remediable weakness into a finding of fraud.

The third trap is the centre of interests. The official guidance warns that nationality may be refused where the applicant lives in France but the spouse or children live abroad. For British families split between a French home and children at school or university in the United Kingdom, this warning bites. The administration looks at where family life, assets and tax ties genuinely sit, and a file that shows a French address but a British-centred life invites rejection. Where the split is temporary and explicable, term-time schooling with full-time reunion in holidays, a spouse completing a fixed posting, the file must say so expressly and prove the French anchoring with tax notices, employment contracts and evidence of daily life. The fourth trap is delay itself. Once the file is complete and the récépissé issued, the eighteen-month period, or twelve months for applicants with ten years of habitual residence, starts to run. If no answer arrives, the applicant should not simply wait. The expiry of the statutory period is the moment to demand a decision in writing and then to enter the formal challenge sequence, because every subsequent remedy runs from dated decisions and strict time limits. In Paris and the Île-de-France, where volumes are heavy and interviews are scheduled through the online platform with the préfecture de police for Paris residents, keeping dated copies of every submission, receipt and summons is essential; files are sometimes completed in stages, and only the receipt for a complete file starts the clock.

For declaration through marriage, refusal takes the form of a refusal to register (refus d’enregistrement), issued by the minister, and the classic traps mirror the three conditions of article 21-2: a community of life that the file fails to prove at the declaration date, a language level below B1, or a marriage whose validity or duration is questioned. The Paris judgments cited in Section I show how judges weigh the evidence: dated, convergent, official documents carry weight, while vague attestations and documents from the wrong period carry none. A British spouse whose declaration is refused registration must act within the short judicial time limits set by the decree of 30 December 1993, seizing the judicial court (tribunal judiciaire) to have French nationality recognised, with the burden of proving that every condition was met on the declaration date.

B. Challenging step by step: the compulsory prior appeal, the Nantes court and the judges who decide

Challenges to naturalisation refusals follow an administrative ladder with an unusual first rung. Anyone whose application for naturalisation or reintegration is declared inadmissible, rejected or adjourned by the préfet must first bring a compulsory prior administrative appeal (recours administratif préalable obligatoire, universally shortened to RAPO) before the Minister of the Interior. The Ministry of the Interior’s own litigation guidance states the rule bluntly: a person who wishes to challenge a prefectoral decision of inadmissibility, rejection or adjournment must first lodge a compulsory prior administrative appeal, the RAPO, with the Ministry of the Interior. Skipping this step and going directly to court guarantees dismissal for failure to exhaust the prior remedy. The RAPO is a full reconsideration, not a mere formality: it is the moment to answer each ground of the refusal with new or better-ordered evidence, a stronger language certificate obtained since the interview, proof of rehabilitation, or documents showing the true centre of family life. It must be filed within two months of notification of the refusal, by a letter that identifies the decision, states the grounds of challenge and attaches the supporting exhibits, sent by a traceable method so the date is beyond dispute.

If the Minister confirms the refusal expressly or by silence, the dispute moves to court, and the court is always the same wherever in France the applicant lives. The Ministry confirms in the same guidance that the administrative court of Nantes alone hears challenges to decisions on naturalisation and reintegration applications, wherever the applicant lives, given the location of the ministerial nationality services. A British applicant living in Paris therefore sues in Nantes, not in the Montreuil administrative court that hears other residence disputes. The same official page maps the rest of the ladder: appeals go to the administrative court of appeal of Nantes, whether brought by the applicant or by the Minister, and the Council of State hears cassation appeals against that court’s judgments. Before the administrative judge, the classic pleas are error of fact, where the administration misread the file, error of law, where it applied the wrong condition, and manifest error of assessment (erreur manifeste d’appréciation), where the conclusion drawn from correct facts is one no reasonable authority could have reached. Adjournments for language level are the ripest target: an applicant who has since obtained a higher certificate, or whose interview report is thinly reasoned, can show that the assessment no longer holds or never held. Rejections based on the centre of interests are fought with the paper trail of French anchoring described above. And where the administration never answered at all, the expiry of the eighteen or twelve-month period in article 21-25-1 frames the argument that the file deserved a decision and that the deemed outcome cannot stand on an empty reasoning.

Disputes about declarations through marriage follow a different track because they belong to the judicial courts (ordre judiciaire), with the public prosecutor as the natural opponent. A refusal of registration is contested before the tribunal judiciaire, which decides whether the declarant is French, and appeals go to the court of appeal, with the Court of Cassation as the final judge of legal correctness. The two Paris judgments and the 2025 Court of Cassation ruling quoted in Section I are the working tools here. They teach three lessons. First, prove the community of life at the declaration date with dated official documents, because courts disregard evidence from the wrong period and vague attestations. Second, prepare the language interview even where age seemed to promise leniency, because the over-sixty exemption concerns diplomas, never knowledge itself. Third, where the State alleges fraud or challenges a registration years later, force it to prove diligence and promptness and answer precisely on arbitrariness and concrete consequences, since the Court of Cassation now reviews these challenges through the two-stage European test of arbitrariness and impact on private life. Conversely, declarants must understand the State’s strongest weapon: the presumption of fraud where common life ends within twelve months of registration, and the prosecutor’s two-year challenge windows, running from registration or from discovery of fraud. Time, in this field, protects the careful and punishes the careless on both sides.

A final practical thread runs through both tracks. Deadlines are short, two months for the RAPO and for most court applications, and they run from notification, so every decision must be read the day it arrives and diarised immediately. Evidence must be dated, official and convergent: tax notices, employment records, school certificates, bills in both spouses’ names, language certificates from recognised bodies. And credibility must be guarded from the first form, because an embellished file discovered at the interview, at the RAPO stage or years later before the courts turns a curable refusal into a fraud finding that follows the applicant across every future application. British applicants who prepare their file the way a judge will read it, condition by condition and date by date, give themselves the best chance at the préfecture and the strongest position in Nantes or before the judicial courts if the administration says no.

Conclusion

Brexit changed the route but not the destination. A British citizen settled in France becomes French either by naturalisation by decree, after five years of habitual residence plus proven assimilation and good character, or by declaration after four years of marriage to a French spouse with an unbroken common life and sufficient French. Both doors are framed by statutes whose words matter: residence at the signature of the decree, the five-year stage and its exceptions, assimilation through language, history and republican values, good morals and the criminal-record wall, the four-year marriage period with community of life at the declaration date, and registration with its two-year challenge windows. Both doors also have locks that a prepared applicant can open: language certificates and a serious interview, a dated paper trail of French family life, a frank criminal-record file, and, when refusal comes, the RAPO within two months, the administrative court of Nantes on appeal to its court of appeal and the Council of State, or the judicial courts for marriage declarations with the fraud and human-rights case law as the frame. Dual nationality is allowed on both sides of the Channel, so nothing about a British past is lost in acquiring a French future. The applicants who succeed are the ones who start early, document everything and treat each condition as a judge would examine it, because in French nationality law the file, more than the person, is what the administration first judges.