You settled in France after Brexit, you hold a Withdrawal Agreement card or a standard British residence permit, and you are starting to ask the next question: should you become French? Since 1 January 2021 British citizens are third-country nationals in France, which means every renewal, every trip and every change of circumstance passes through the prefecture. French nationality ends that cycle. It gives you the right to live, work and vote in France without a card, to move freely across the European Union again, and to pass a secure status to your children. The price is a demanding procedure: years of stable residence, proof that your life is genuinely rooted in France, a language test pitched well above holiday French, and an administration that refuses or postpones a large share of applications. This guide explains the two routes that matter for British readers, naturalisation by decree after years of residence and declaration through marriage to a French spouse, what holding both passports really changes, and exactly how to challenge a refusal, an adjournment or an opposition before the courts. If you are still at the arrival stage, start with our guide to arriving and staying in France as a British citizen, then return here once residence is secure. Readers focused on the decree route alone can also consult our companion guides on five years’ residence, assimilation and language and on becoming French by naturalisation; the present guide adds the marriage-declaration route, dual nationality and children.

I. How does a British citizen become French after Brexit?

A. Naturalisation by decree: five years of residence and the assimilation test

Naturalisation by decree, called naturalisation par décret, is the standard route for a British citizen with no French spouse or French parent. It is a favour granted by the State, not a right earned automatically, and the statute says so in one sentence. 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. » Nationality through this route comes from a decree issued at your request, which means the administration enjoys wide discretion and the judge reviews refusals with restraint. Prepare the file with that reality in mind: every gap in residence, every undeclared tax year and every weak language result gives the prefect a lawful reason to say no.

The first hurdle is time. Article 21-17 of the Civil 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. » Five years of habitual residence before the application is the default. For most British applicants the clock started on arrival after Brexit, so an application filed in 2026 typically needs continuous French residence since 2021, proved by tax notices, rental contracts, utility bills and health cover. The five-year period can fall to two years in defined cases, and article 21-18 of the Civil Code lists them, including two years of successful higher education towards a French degree and a record of exceptional integration in civic, scientific, economic, cultural or sporting life. A British graduate of a French university or a researcher with a recognised record should check these reductions before waiting out the full five years.

Residence must also exist on the day the decree is signed, not only on the day you apply. Article 21-16 of the Civil Code provides: « Nul ne peut être naturalisé s’il n’a en France sa résidence au moment de la signature du décret de naturalisation. » A move back to London, even temporary-looking, between the application and the decree can sink the file. British applicants who travel constantly for work should keep their centre of interests visibly in France throughout the procedure: the French home as the family home, French tax residence, children schooled in France, and a paper trail to prove it. The prefecture re-checks residence before the decree, and files have been refused at that late stage because the applicant had relocated without saying so.

The second hurdle is assimilation, called assimilation à la communauté française, which covers language, knowledge of France and attachment to republican values. Article 21-24 of the Civil Code states: « 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 language standard now sits at a level where you must, in the words of the same article, understand « le contenu essentiel de sujets concrets ou abstraits dans un texte complexe », communicate spontaneously and express yourself clearly and in detail on a wide range of subjects. In practice this is tested through an oral interview at the prefecture and, increasingly, a certified diploma at B1 level or above. Holiday French will not carry you through; many British refusals turn entirely on ten minutes of hesitant answers in a prefecture office. At the end of the assimilation check you sign the charter of rights and duties of the French citizen, the charte des droits et devoirs du citoyen français, which the article describes as recalling « les principes, valeurs et symboles essentiels de la République française ».

The third hurdle is conduct and paperwork honesty. Article 21-23 of the Civil Code bars anyone who is not of « bonnes vie et moeurs », of good character, or who has suffered one of the convictions listed in article 21-27. And article 21-27 of the Civil Code goes further: « Il en est de même de celui dont le séjour en France est irrégulier au regard des lois et conventions relatives au séjour des étrangers en France. » Irregular stay alone disqualifies you. For British applicants this is a very practical warning: a gap between cards, an expired Withdrawal Agreement permit renewed late, or a period spent in France on a visitor visa while working remotely can all be read as irregularity. Regularise first, document every period, and only then apply. Tax regularity matters just as much. The prefecture consults the tax office, and undeclared French rental income or missing returns routinely produce adjournments, called ajournements, of two or three years.

B. Married to a French spouse: the four-year declaration route

Where your husband or wife is French, the route is different and often faster: acquisition by declaration, called acquisition par déclaration, rather than by decree. 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. » Four years of marriage, a shared life that has never stopped, and a spouse still French on the day of the declaration: those are the three pillars. The period rises to five years where you cannot show three years of uninterrupted and lawful residence in France since the wedding, or where your French spouse was not registered on the consular roll of French citizens abroad during your life together overseas. British couples who married in England and lived in London before moving to France should verify these residence and registration conditions early, because they are the most common trap on this route.

Shared life, called communauté de vie tant affective que matérielle, means exactly what it says: an emotional and a material life in common, maintained without interruption from the wedding day to the declaration day. The administration checks cohabitation through joint tax returns, the shared lease or title deeds, joint bank accounts, birth certificates of children, and sometimes a home visit or a hearing. A separation, even brief and even reconciled, can be fatal if it falls inside the reference period. Courts apply this strictly. In a judgment of 17 September 2026 (RG 22/03141), the judicial court of Nantes recalled that « Or, l’article 21-2 du code civil exige qu’à la date de la déclaration de nationalité française, soit en l’espèce le 24 juillet 2019, la communauté de vie n’ait pas cessé depuis le mariage. », before finding that « la communauté de vie entre les époux ne peut être considérée comme continue depuis le mariage au moment de la déclaration » where the couple’s history showed infidelity and criminal proceedings between them, and concluding: « Dit que Madame [C] [S], se disant née le 7 octobre 1980 à [Localité 4] (Cameroun), n’est pas de nationalité française; » See Tribunal judiciaire de Nantes, 17 September 2026, RG 22/03141. The lesson for British spouses is blunt: do not file while separated, while divorce is discussed, or while living apart for work without strong proof that the household remains common. Wait until cohabitation is solid and documented, then declare.

Language matters on this route too, and the standard has teeth. The foreign spouse must show sufficient knowledge of French, assessed against a set level, and refusals to record the declaration on language grounds are routinely upheld. In a decision of 31 January 2023 (RG 21/12332), the Paris Court of Appeal confirmed a refusal where « le ministre de l’intérieur a rejeté la demande d’enregistrement de cette déclaration au motif que M. [R] [O] ne justifiait pas d’une connaissance orale suffisante de la langue française, son niveau vérifié par un agent de la préfecture étant inférieur au niveau B1 oral du cadre européen commun de référence pour les langues », adding that the burden lay on the declarant to prove he met the conditions of article 21-2 on the day of his declaration. See Cour d’appel de Paris, 31 January 2023, RG 21/12332. A British spouse who speaks French only at home should therefore prepare the assessed oral level seriously, ideally with a certified test before filing, because the prefecture interview is short, formal and decisive.

Procedure runs through the prefecture of your home address, or the French consulate if you live in the United Kingdom. Article 26 of the Civil Code explains that marriage-based declarations are received by the administrative authority and that « Il en est délivré récépissé après remise des pièces nécessaires à la preuve de leur recevabilité. » That receipt, the récépissé, starts the clock: from its date the State has a limited time to refuse registration for lack of conditions, and the Government has two years to oppose the acquisition. Article 21-4 of the Civil Code states: « 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 ». Opposition for unworthiness or lack of assimilation, other than language, within two years of the receipt. Polygamy and certain convictions for sexual offences against young children are deemed by the same article to constitute lack of assimilation. If no refusal and no opposition arrives in time, the declaration is recorded and you are French from the date of the declaration itself, which is why keeping the receipt safe matters so much. British applicants living in France file at the prefecture platform of their département; in Paris that means the Paris prefecture’s online nationality desk, with an in-person interview later. Allow twelve to twenty-four months from filing to recording in straightforward cases, longer where enquiries are ordered.

II. What changes once you are French, and how do you challenge a refusal?

A. Two passports, British children and daily life with French nationality

France accepts dual nationality without reservation, and the United Kingdom does the same. The British government’s official guidance confirms that you may hold British and French citizenship at once, which means naturalisation does not require you to renounce your British passport or to choose between consular protections. See gov.uk: dual citizenship. You travel on either passport, you keep your right of abode in the United Kingdom, and your British pension, property and inheritance position under British law is unchanged by the French decree. What changes is your position in France and in Europe: no more residence card, no more prefecture renewals, the right to vote in all French elections including parliamentary and presidential ones, access to public employment reserved to nationals, and the restoration of European Union free movement for work or retirement in Spain, Italy or Ireland. For a British family settled in the Dordogne or working in Paris, that combination of a British passport for London and a French passport for everything else is often the whole point of the exercise.

Children need separate attention because French nationality does not cascade automatically in every direction. A child born in France to British parents does not become French at birth, but the route opens later: article 21-11 of the Civil Code lets a foreign-born child raised in France claim French nationality by declaration from age sixteen, or through the parents from age thirteen, where residence conditions measured from age eleven, or eight for the earlier application, are met. Meanwhile your own naturalisation extends its effects to your minor children who live with you, provided their names appear in the decree, which is why the application form asks for each child’s details and why omitting a child from the file is a costly mistake. British parents should therefore plan the family as a unit: list every minor child living in France, attach their birth certificates with sworn translations, called traductions assermentées, and their school certificates, and check that each child’s residence is documented year by year. An adult child must apply separately, on their own residence and assimilation record.

Nationality also reshapes two subjects this desk covers regularly: succession and taxation. On succession, becoming French does not erase the European Succession Regulation, which continues to point by default to the law of your habitual residence, but it changes the practical landscape around the French reserved share, the réserve héréditaire, and around gifts already made under English law. Review any English will and any choice-of-law declaration after the decree with a practitioner who handles both systems; our guide to British wills and French houses explains the moving parts. On tax, French nationality alone does not make you French tax resident, and British nationality alone does not keep you British tax resident: residence, not the passport, drives the France-United Kingdom double tax treaty. Keep filing in the country where you are resident, keep the treaty tie-breakers in mind if you split time, and do not assume the decree changes which treasury taxes your pension. What the decree does change is administrative: French citizens deal with the tax office, the health system and the town hall without ever proving a right of residence again, which removes an entire layer of friction from yearly paperwork.

One Paris and Île-de-France note, because most British professionals naturalise through the capital’s prefecture. The Paris procedure is fully file-based at the start, with the interview scheduled months after the acknowledgment, and Paris examiners are known for rigorous language interviews and systematic tax-office checks. Build the Paris file accordingly: certified French test result enclosed from the outset rather than promised later, five complete tax years with all notices, called avis d’imposition, and employer certificates covering the whole period. Applicants elsewhere in the Île-de-France file at their département prefecture under the same national rules, but waiting times differ, so ask for the local indicative timetable at the counter and diary the follow-up. Wherever you file, keep a complete copy of everything sent, because lost-document disputes are far easier to win when you can reproduce the file page for page.

B. Refusal, adjournment and taking the decision to court

Adverse decisions come in three shapes, and each has its own reply. A refusal, called refus or rejet, closes the file on a ground such as insufficient language, unstable resources or weak integration. An adjournment, the ajournement, postpones the decision by two or three years to let you consolidate residence, income or language, and bars any fresh application before the period expires. An opposition, the opposition du Gouvernement on the marriage route, blocks the declaration within two years of the receipt for unworthiness or lack of assimilation. Read the notification letter word by word on arrival: the legal label printed on it decides the deadline, the court and the argument. A refusal grounded on language calls for a certified test result and a request for a fresh interview; a refusal grounded on resources calls for reconstructed accounts; an adjournment calls for patience plus evidence-building rather than an immediate frontal attack, because courts rarely annul a postponement that gives reasoned guidance. Never refile an identical dossier after a refusal without fixing the stated ground: the second refusal will simply quote the first, and the judge will see a file that ignored the administration’s directions.

Start every challenge with the informal appeal, which carries no filing charge. A recours gracieux asks the minister or the prefect to reconsider, and sometimes reverses refusals built on a missing or misread document: a diploma the office overlooked, a tax notice filed under a maiden name, a language certificate that arrived after the interview. Send it by registered post with acknowledgment, called lettre recommandée avec accusé de réception, within two months of notification, enclose the missing proof, and keep the slip. But never let the informal appeal consume the court deadline. Naturalisation refusals are challenged before the administrative court, the tribunal administratif of your home address, within two months, with a pleading that attacks the legal basis, the factual accuracy and the proportionality of the decision. Refusals to record a marriage declaration, and oppositions to it, go instead before the civil courts, the tribunal judiciaire, which decides whether the conditions of article 21-2 were truly met on declaration day, as the Nantes and Paris rulings above illustrate. Filing in the wrong court wastes the deadline, so match the forum to the route: administrative judge for the decree, civil judge for the declaration.

Plead the way successful applicants plead. Against a language refusal, produce a certified result at or above the required level dated as close as possible to the interview, plus evidence of daily French use: employment in French, children’s schooling correspondence, association life. The Paris ruling shows why this matters: the court upheld the refusal precisely because the assessed oral level sat below B1 and the declarant could not prove otherwise. Against a shared-life refusal, rebuild cohabitation month by month with joint documents spanning the whole period from marriage to declaration, and explain every anomaly, a posting abroad, a hospital stay, a separate lease during works, with dated proof rather than assertions. Against an resources refusal, present five years of tax notices, employment contracts and bank statements as a single table with yearly totals, and reconcile every dip with maternity leave, study, or a documented career break. Against an adjournment, show the guidance has been followed: new diploma obtained, stable contract signed, tax arrears cleared. Judges decide nationality cases on files, not on speeches, and the file that answers the decision point by point defeats the file that repeats generalities about integration.

While the case runs, protect your residence position in parallel. Challenging a nationality refusal does not by itself extend a residence card, so renew your British permit on its own timetable, keep every receipt, called récépissé, and never let a gap open that the administration could later read as irregular stay under article 21-27. If a removal order, an obligation de quitter le territoire français, is notified alongside a residence refusal, calendar its much shorter deadlines immediately and seek interim relief, because some removal measures can be executed once deadlines expire. Keep living the file you are defending: stay in France, keep working, keep learning French, file taxes early. Fresh progress after the refusal, a higher language certificate, a permanent contract, a completed tax regularisation, can support a new application even where the court case is lost, and prefectures look kindly on applicants who return with the exact evidence the refusal asked for. Nationality rewards the long, documented, steadily improving file; it punishes haste, gaps and repeated identical filings.

Conclusion

For a British citizen settled in France, nationality is the natural end of the Brexit journey: the decree route after five years of rooted, lawful, tax-clean residence with real French, or the declaration route after four years of unbroken shared life with a French spouse. Both demand the same discipline, continuous proof of home, money and language, a file that anticipates the examiner’s doubts, and prompt, forum-correct challenges when the answer is no. The rewards match the effort. Two passports, secure status for children named in the decree, full political rights in France, renewed European mobility, and an end to prefecture renewals. Start early, test your French before the administration does, keep every tax notice and every joint bill, and treat any refusal as a work plan rather than a verdict. Handled that way, the British passport that once complicated your French life becomes, paired with a French one, its strongest asset.