Since the end of the Brexit transition period, British citizens living in France are third-country nationals. A Withdrawal Agreement residence permit, commonly called the WARP card (the carte de séjour issued under the EU-UK Withdrawal Agreement), secures your right to stay, but it does not make you French. For Britons who have built their working life, paid their taxes and raised children in France, the next question arrives naturally: can you become French, keep your British passport, and what happens if the administration says no? The answer is yes in principle. France allows dual nationality, and the United Kingdom does too. The route most British residents use is naturalisation par décret, which means naturalisation by decree: a discretionary grant of French nationality by the French government after at least five years of habitual residence, proof of assimilation into the French community, and a formal investigation of your file. The procedure is now filed online, the administration has a maximum of eighteen months to answer, and an unfavourable decision can be challenged, first before the minister in charge of naturalisations and then before the administrative court (tribunal administratif) of Nantes, which has nationwide jurisdiction over these disputes. This guide explains, with the exact legal texts and recent court decisions, how long you must have lived in France, what the language and assimilation test really requires, how a refusal or adjournment can be overturned, and when marriage to a French citizen offers a faster declaration route instead.
I. Can a British citizen be naturalised French after Brexit, and what must you prove?
Naturalisation is not a right that follows automatically from years spent in France. It is a favour granted by decree, and the minister enjoys what the courts call a wide discretion. That makes preparation decisive: most refusals punished files that were filed too early, with weak evidence of professional integration, or with an underestimated language interview. The two pillars of every successful application are residence and assimilation.
A. How long must a British applicant have lived in France before applying for naturalisation?
The starting rule is simple and strict. Article 21-15 of the Civil Code provides: “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: except for one special case, acquiring French nationality by decision of the public authority results from a naturalisation granted by decree at the foreigner’s request. You must therefore apply; nothing is automatic, however long you have lived in France.
The residence condition has two limbs. First, at the moment the decree is signed you must actually live in France. Article 21-16 of the Civil Code states: “Nul ne peut être naturalisé s’il n’a en France sa résidence au moment de la signature du décret de naturalisation.” Nobody may be naturalised unless they reside in France when the naturalisation decree is signed. A British applicant who moves back to London while the file is pending therefore sinks the application.
Second, you must show five years of habitual residence before filing. 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.” The official English guidance on service-public confirms the same rule: a minimum of five years of residence in France is required For British residents, years lived in France before Brexit count fully, including years covered first by EU free movement and then by a Withdrawal Agreement card, because what matters is habitual, lawful residence, not the label on the permit. If you hold a WARP card and wonder how renewals and travel work while your file is pending, the rules on renewing a Withdrawal Agreement residence permit and challenging a refusal are directly relevant, since you must hold a valid residence permit when you file and remain lawfully resident throughout the procedure.
Three sets of exceptions shorten or remove the five-year stage. The two-year reduction in article 21-18 of the Civil Code covers foreigners who have completed two years of higher education leading to a French degree, those who have rendered or can render important services to France through their abilities and talents, and those with an exceptional integration record in civic, scientific, economic, cultural or sporting life. Full exemption from any stage under article 21-19 of the Civil Code covers, among others, foreigners who have served in the French army, those who have rendered exceptional services to France or whose naturalisation is of exceptional interest to France, with a reasoned opinion of the Council of State, and recognised refugees. And article 21-20 of the Civil Code exempts members of the French cultural and linguistic community who come from French-speaking countries, speak French as their mother tongue, or prove five years of schooling in French. A British applicant educated at a French lycée abroad for five years can therefore rely on this last provision. Article 21-26 of the Civil Code adds that certain stays outside France count as residence in France, notably service for the French State or a body of particular interest to the French economy or culture.
Residence alone is never enough. The administration checks where the centre of your family life sits: the official guidance warns that applicants whose spouse or children live abroad may be refused French nationality A British executive posted in Paris whose wife and children remained in Surrey must therefore be ready to show why France is genuinely the family home, with school enrolment, tax household and daily life to prove it. Students face a related trap: years spent in France on a student permit count as residence, but without subsequent stable employment the file usually fails on professional integration, examined below.
B. What does French assimilation mean for a British applicant: language, values and the prefecture interview?
Assimilation (assimilation à la communauté française) is the legal term for proving that you have become part of French society. Article 21-24 of the Civil Code, as quoted by the Nantes appeal court in a 2020 naturalisation judgment, 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.” At the end of the assimilation check, the applicant signs the charter of rights and duties of the French citizen (charte des droits et devoirs du citoyen français), which restates the essential principles, values and symbols of the French Republic. Concretely, assimilation is tested on four fronts: language, knowledge of France and its values, professional integration, and conduct.
Language comes first and causes the most refusals. The level required is B1 of the Common European Framework of Reference for Languages, assessed through an interview at the prefecture (entretien d’assimilation) and through diplomas or certificates. The 2020 Nantes judgment (CAA Nantes, 2nd chamber, 2 April 2020, No 19NT04015) quotes the implementing decree in these terms: the required level is “Son niveau est celui défini par le niveau B1”, measured across listening, taking part in conversation and continuous speaking under the Council of Europe Common European Framework. The current wording of the decree tests both spoken and written French, and applicants are expected to produce a diploma or certificate at the required level unless an exemption applies, for example on medical grounds. The interview itself is scored against a formal grid set by the order (arrêté) of 22 February 2005, and the courts enforce that grid strictly.
A 2 April 2020 judgment of the Nantes administrative court of appeal shows how unforgiving this test is. The case, CAA Nantes, 2nd chamber, 2 April 2020, No 19NT04015, concerned a woman whose application had been declared inadmissible for lack of linguistic assimilation. The minister had found that during the prefecture interview she had been unable to understand the essential points of an ordinary conversation or to converse on familiar subjects connected with her interests, and the language assessor had concluded that the required B1 level was not reached. She argued that the grid had not been properly applied, that she managed everyday life in French, that her husband and daughter were French, and that illness had affected her performance. The court rejected every argument. It held that the evaluation grid had indeed been used, that she did not allege the questions were inappropriate, and that family testimonials about daily errands could not outweigh the interview record. Its conclusion deserves quotation in full because it states the law as it still applies today: “eu égard au large pouvoir dont il dispose, le ministre a pu, sans commettre d’erreur de fait ou d’erreur d’appréciation, déclarer irrecevable la demande de naturalisation de Mme F… pour défaut d’assimilation linguistique.” Given the wide power available to him, the minister could, without any error of fact or error of assessment, declare the application inadmissible for lack of linguistic assimilation. British applicants should take three lessons from this. First, conversational survival French is not B1: you must follow a sustained conversation and speak coherently about your life, work and interests. Second, prepare for the interview as an examination, not a chat, and consider certified B1 courses and mock interviews before filing. Third, the minister’s assessment enjoys a wide margin, so the file must make refusal difficult rather than invite a second chance.
Knowledge of France and republican values is examined in the same interview. You are expected to know the organisation of the French Republic, its history, its symbols, and the rights and duties French nationality confers, and to demonstrate genuine adherence to the Republic’s essential principles and values. Signing the charter is not a formality: contradictions between your statements and the charter’s principles can ground a refusal.
Professional integration is, in the administration’s own words, an essential condition. The guidance states that it means holding stable and sufficient income to support yourself and your family tax household (foyer fiscal), and the courts allow the minister to weigh it heavily. A 29 November 2021 judgment, CAA Nantes, 2nd chamber, 29 November 2021, No 20NT03860, is the leading illustration. The applicant had lived in France since 2010, had been genuinely and constantly mobilised to integrate, and had signed an open-ended full-time contract in March 2018. The lower court had annulled the two-year adjournment of his application. The court of appeal reversed, recalling first that “L’autorité administrative dispose, en matière de naturalisation ou de réintégration dans la nationalité française, d’un large pouvoir d’appréciation.” The administration has, in naturalisation matters, a wide discretion. It then held that although the applicant had to be regarded as permanently hired from March 2018, that situation was still recent when the decision was taken, and his earlier career had been discontinuous and precarious. Consequence: “le ministre de l’intérieur n’a pas, en estimant que M. B… n’avait pas pleinement réalisé son insertion professionnelle, entaché sa décision d’erreur manifeste d’appréciation.” By finding that the applicant had not fully achieved professional integration, the minister had not tainted his decision with a manifest error of assessment. For British readers the message is blunt: file when your employment is stable and settled, ideally with at least a year or two of continuous work behind you, clean tax returns, and payslips showing durable income. Freelancers, recent contractors and applicants between jobs should wait or document exceptional circumstances.
Conduct and loyalty close the list. A criminal record can bar naturalisation outright: under article 21-27 of the Civil Code, naturalisation is closed to anyone where “s’il a été condamné à une peine égale ou supérieure à six mois d’emprisonnement, non assortie d’une mesure de sursis”, as well as persons under an unlifted expulsion order or living in irregular stay. Tax compliance, absence of fraud, and loyalty to France are examined through police and administrative inquiries. Unpaid taxes, undeclared work or driving offences repeated often enough to show disregard for the law can each tip a borderline file into refusal.
II. What happens if the administration says no, and is marriage a faster route to French nationality?
A refusal is not the end of the road, but the clock starts immediately and the remedy depends on the exact type of decision. French practice distinguishes three unfavourable outcomes: inadmissibility (irrecevabilité), where a legal condition is missing; rejection (rejet), where the minister finds no reason to grant nationality; and adjournment (ajournement), where the application is postponed, typically for two years, to let you stabilise your employment or integration. An adjournment is the least severe: once the period expires you may file a fresh application. A rejection is harsher: the guidance warns that a new application filed less than five years after notification of a rejection for inopportuneness may be closed without examination. Identifying which decision you received is therefore the first step, and the decision letter must state the remedies and time limits.
A. How do you challenge a refusal, adjournment or inadmissibility decision: hierarchical appeal and the Nantes administrative court?
Begin with the timetable. The administration has eighteen months from the issue of the receipt (récépissé) to answer a naturalisation application, reduced to twelve months if you prove ten years of habitual residence in France, with one possible three-month extension that must be reasoned. If the answer is negative, you have two months from notification to lodge a hierarchical appeal (recours hiérarchique) to the minister in charge of naturalisations. Files sent online must use the form in the personal notification area; files sent by post follow the paper route. If the minister confirms the refusal expressly, or stays silent for four months, which counts as an implied rejection, you then have two months to bring a court action (recours contentieux) before the administrative court of Nantes, the single court with jurisdiction over naturalisation disputes. The 2 April 2020 Nantes judgment explains an important procedural subtlety: under article 45 of the decree of 30 December 1993, the minister’s decision on the preliminary appeal replaces the prefect’s decision, so your court action must target the minister’s decision, not the earlier prefectural one.
Before the court, the standard of review is the key to strategy. The minister has a wide discretion, so the judge does not substitute a personal view of whether you deserve nationality. Against a rejection or an inadmissibility decision, the court checks for errors of fact, errors of law and errors of assessment; against an adjournment, which is a softer measure, review is limited to manifest error (erreur manifeste d’appréciation). The 2021 reversal in CAA Nantes, No 20NT03860 shows how high that bar sits: even a sympathetic record of constant effort since 2010 and a fresh permanent contract did not make a two-year adjournment manifestly wrong. Conversely, decisions fall when the facts are wrong, when the grid was not applied, when the reasoning contradicts the file, or when the administration relied on a ground the law does not allow. A very recent judgment confirms the framework is unchanged: CAA Nantes, 2nd chamber, 20 March 2026, No 25NT01434 recalls that under the third paragraph of article 48 of the decree of 30 December 1993, “Si le ministre chargé des naturalisations estime qu’il n’y a pas lieu d’accorder la naturalisation ou la réintégration sollicitée, il prononce le rejet de la demande.” If the minister considers there is no reason to grant the requested naturalisation, he rejects the application. The court then measures that rejection against article 21-24 and the language decree, exactly as in the earlier cases.
Practical advice for British applicants in Paris and the Île-de-France follows from this case law. File online through your personal area and keep every notification, because the two-month appeal time runs from notification. Treat the prefecture interview as the decisive moment: request the account of the interview, check it against what was actually asked, and challenge any gap between the grid and the report. If you receive an adjournment for professional reasons, use the waiting period as the court expects: continuous employment, rising or stable income, full tax compliance, and, where relevant, improved French certified by a new diploma. If you receive a rejection, do not simply refile identically within five years; address each ground with new evidence, because a repeat file without new circumstances can be closed without examination. And never let a deadline pass unchallenged: an unappealed refusal becomes final, while a well-pleaded hierarchical appeal often leads the ministry to re-examine the file before any judge is involved. Applicants based in Paris attend their interview and any citizenship ceremony at the Paris services, but every dispute converges on Nantes, so instruct counsel used to that court’s demanding but predictable review.
B. Can a British spouse of a French citizen use the four-year marriage declaration instead?
Marriage to a French citizen opens a different door: acquisition by declaration (acquisition par déclaration à raison du mariage), which is a claim of right rather than a discretionary favour, though the government can still oppose it on limited grounds. Article 21-2 of the Civil Code sets the conditions: “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 citizen may, after four years from the marriage, acquire French nationality by declaration, provided the shared life, both emotional and material, has continued without interruption since the marriage and the French spouse has kept French nationality. The same article extends the period of shared life to five years where the foreigner cannot show three years of uninterrupted lawful residence in France since the marriage, or cannot prove the French spouse’s registration abroad, and it adds two further requirements: a marriage celebrated abroad must first have been transcribed onto the French civil-status registers, and the foreign spouse must show sufficient knowledge of French. For a British husband or wife living in France, the four-year route is therefore realistic once three years of French residence are documented; for a couple living in London, the five-year route applies and transcription of the English marriage certificate at the French consulate becomes the critical preliminary step.
Declaration is faster and more predictable than naturalisation, but it is not unchecked. The prefecture investigates the reality of the shared life and the language level, and the government may oppose the acquisition by decree where the applicant lacks assimilation or has disqualifying convictions, with judicial review available against that opposition. Sham or broken marriages are the main cause of failure: separation, even without divorce, breaks the required community of life. Genuine couples should file with joint tax notices, a shared lease or title deeds, joint bank records and consistent evidence of daily life together.
Two final questions concern every British family. First, dual nationality: becoming French does not cost you your British citizenship. The United Kingdom’s official guidance on dual citizenship confirms that becoming French does not cost a British citizen their British citizenship. France likewise permits its new citizens to keep their previous nationality, so a naturalised Briton votes, works and travels as a French citizen while remaining British. Second, the children: a minor child who remains foreign when a parent becomes French may be granted naturalisation with the parent if the child has lived in France with that parent for five years before the application. Readers expecting a child or managing mixed-nationality children should also check the distinct rules on French and British nationality for babies born in France to British parents, because birth in France alone does not make a child French.
Conclusion
For a British resident, French naturalisation by decree rewards five years of genuine French life: lawful residence with your family interests in France, B1 French demonstrated in a demanding interview, stable professional integration, and clean conduct. The minister’s wide discretion means the file must be complete before it is filed, not repaired afterwards. If the answer is no, identify whether you face inadmissibility, rejection or adjournment, lodge the hierarchical appeal within two months, and take the case to the Nantes administrative court where the decision is legally vulnerable. Where a French spouse shares your life, the four-year marriage declaration may offer the shorter path. In each route, British citizenship is preserved alongside French nationality, and the effort invested in language, employment and evidence is what turns a hopeful application into a decree published in the Official Journal.