You married a French citizen, you live all or part of your life in France, and since Brexit you have felt the difference every time you queue in the non-EU line, renew a residence permit or explain your status to an employer. Becoming French would end most of that friction at a stroke: no more residence cards, a French passport, the right to vote in every election and the freedom to come and go as you please. For the British husband or wife of a French national, the fastest route is rarely the well-known naturalisation by decree. It is a quieter procedure called the déclaration de nationalité à raison du mariage, the nationality declaration on grounds of marriage: after four years of marriage you declare that you are French, and you become French, unless the State finds a legal reason to stop you. This guide explains the whole route in plain English. It sets out who qualifies, which documents prove it, what happens at the joint interview at the préfecture (the local office of the State administration), and the two very different ways the administration can block you: the reasoned refusal to register your declaration, which you fight before the civil courts, and the Government opposition decided by decree, which you fight before the Conseil d’État (the supreme administrative court). Every decisive rule is quoted from the statute or the decided cases, because in nationality law the exact wording is what wins or loses.

I. Who can become French by marriage declaration and what file proves it

Marriage to a French citizen does not make you French automatically. It gives you the right to claim French nationality by a formal declaration, provided four cumulative conditions are met on the day you sign it. The rule sits in article 21-2 of the Civil Code, which states: “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.” In other words, a foreigner who marries a French national may acquire French nationality by declaration after four years from the wedding, on condition that on the date of the declaration the shared life, both emotional and material, has not ended since the marriage, and that the French spouse has kept French nationality. Three features of this route matter enormously for British applicants. First, there is no discretionary judgment about whether you “deserve” to be French, as there is in naturalisation by decree: if the legal conditions are met, the declaration must be registered. Second, you do not need five years of residence in France, and you do not even need to live in France at all, as long as the residence or registration conditions described below are satisfied. Third, Britain imposes no obstacle on its side: the British government confirms that “Dual citizenship (also known as dual nationality) is allowed in the UK. This means you can be a British citizen and also a citizen of other countries,” and that “You do not need to apply for dual citizenship. You can apply for foreign citizenship and keep your British citizenship” (gov.uk, Dual citizenship). You keep your British passport and add a French one.

A. Four years of marriage, five in some cases, and a shared life that must still exist on declaration day

The clock starts on the wedding day, and the basic delay is four years of marriage before you may sign the declaration. The statute then extends the required period of communauté de vie (shared married life) to five years in one situation: where, at the moment of the declaration, you cannot show either three years of continuous lawful residence in France since the marriage or proof that your French spouse was registered throughout your shared life abroad on the registre des Français établis hors de France (the register of French nationals living abroad). Concretely, a British spouse who moved to Lyon the year after the wedding and has lived there lawfully ever since declares after four years; a British spouse who stayed in London while the French husband or wife never registered at the French consulate waits five. If the wedding took place outside France, it must first have been transcribed, which is the transcription procedure by which a foreign marriage certificate is copied onto the French civil-status registers; without that prior transcription no declaration is possible. And the French spouse must still be French on declaration day: if your husband or wife has in the meantime lost or renounced French nationality, the route is closed.

The condition that destroys the most files is the communauté de vie tant affective que matérielle, the shared life that must be both emotional and material, unbroken since the wedding and still alive on the day you declare. The administration does not ask whether you love each other in the abstract; it checks addresses, bills, tax notices, children and daily life, and the courts give the word a demanding meaning. The leading decision is Court of Cassation, First Civil Chamber, 10 February 2021, appeal no. 20-11.694. A man married to a French woman had signed a marriage-based declaration in 2009, then divorced in 2013 and remarried in Switzerland; the public prosecutor sought annulment of the registration on the ground that the declarant had fathered two children with his future second wife while still married. The Court rejected his appeal and approved the finding that there had been no emotional shared life on the day of the declaration: “la cour d’appel a souverainement déduit, sans se fonder uniquement sur la circonstance que M. B… entretenait une relation adultère, qu’il n’existait pas de communauté de vie affective des époux au jour de la déclaration souscrite le 3 avril 2009, de sorte que la fraude était caractérisée.” The appeal court had sovereignly deduced, without relying solely on the fact of the adulterous relationship, that no emotional shared life existed between the spouses on the day of the declaration, so that fraud was established. Note the nuance the Court preserves: an affair alone does not automatically kill the shared life, but a parallel durable emotional life with children by another partner does. For British couples the practical lessons are blunt. A separation, even presented as temporary, a second household abroad that looks like the real home, or children conceived with someone else during the marriage can each be treated as the end of the shared life. And the danger does not stop at registration: article 26-4 of the Civil Code provides that “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” — ending the shared life within twelve months after registration creates a legal presumption of fraud, which the public prosecutor can use to have the registration annulled years later. Sign nothing until the marriage is genuinely solid, and if a separation happens after you declare, take advice immediately rather than hoping nobody notices. You must also tell the préfecture of any change in your situation after filing, such as a move or a divorce, because the file is judged on the facts as they stand, not as they stood when you posted it.

B. The French paperwork trail, the B2 French test and the joint interview

The file you build is heavy, and every missing or stale document is a ground for delay. The official checklist on service-public.fr, acquiring French nationality by marriage, requires your full birth certificate showing your parents’ names, a marriage certificate less than three months old — or, for a wedding celebrated abroad such as a British ceremony, a copy of its transcription onto the French consular registers, also less than three months old and proving a marriage contracted at least four years earlier — plus the dissolution papers for any previous marriages and the birth certificates of all your children, including children born before the wedding. Where you married less than five years ago, add either proof of three years of continuous lawful residence in France since the marriage, meaning residence permits, receipts for permit applications, employment contracts, pay slips, energy bills and tax documents, or, if you lived abroad, the certificate that your French spouse was registered on the register of French nationals living abroad throughout your shared life there. British applicants living in France prove lawful residence with their post-Brexit permit, most often the Withdrawal Agreement card whose renewal rules are explained in this guide to renewing a British WARP residence card in France. If you lived in any country for more than six months during the last ten years, you must also produce a foreign criminal-record extract issued after your arrival in France for each such country. Every document in English needs a translation by a traducteur agréé (sworn translator authorised to act before the courts or the administration), and some foreign documents must carry an apostille or be légalisés (legalised, meaning authenticated by the authorities for use abroad); ask the French consulate which formality applies to each British certificate before you pay for anything. The procedure itself costs nothing beyond the timbre fiscal (the electronic tax stamp), and beware of private websites selling paid appointments: the administration stresses that acquisition of French nationality is a free process. If your file is incomplete, the office sets you a deadline to complete it, and silence leads to classement sans suite (closure without examination), which you can challenge within six months and which never stops you filing a fresh declaration.

Language is the second great filter, and the bar has risen. You must now prove French at level B2 of the Common European Framework, both spoken and written: service-public.fr, required level of French for French nationality, states that “Vous devez justifier d’une connaissance de la langue française à l’oral et à l’écrit au moins égale au niveau B2”. Acceptable proof includes a French diploma at the required level or a pass certificate for the test de connaissance du français (TCF, run by France Éducation International) or the test d’évaluation de français (TEF, run by the Paris Chamber of Commerce and Industry), each valid for less than two years. A rusty school certificate will not do, and the many British applicants who sailed through the old conversational check should budget for real preparation: B2 means handling an administrative meeting, a disagreement and a formal letter in French. The file ends with a joint interview to which both spouses are summoned. Officers verify that the shared life is real and that you are assimilated into French society, you show your original identity documents, and you sign an attestation sur l’honneur (a sworn written statement) that your emotional and material shared life has continued since the wedding. Assimilation here means accepting the essential principles and values of the Republic; the official guidance warns that conduct revealing discrimination based on sex, race, religion or nationality counts as failed assimilation. Once the interview is over you receive a récépissé (an official receipt for your complete file), and the legal clocks start running from that date. If the declaration is registered, your French nationality takes effect retroactively on the date you signed it, and you may at the same time request francisation (the French rendering of your first name or surname) for yourself and your children. Two absolute bars should be checked before you start: article 21-27 of the Civil Code excludes anyone sentenced to six months’ imprisonment or more without suspension — “s’il a été condamné à une peine égale ou supérieure à six mois d’emprisonnement, non assortie d’une mesure de sursis” — and anyone whose stay in France is irregular: “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.” Keep your residence status spotless while the file is pending.

II. Refusal to register and State opposition: two attacks, two courts, two strategies

Once your receipt is issued, the administration has one year to refuse registration of a marriage-based declaration, extended to two years if the Government opens an opposition procedure; the rule is in article 26-3 of the Civil Code, which adds that “Sa décision motivée est notifiée au déclarant qui peut la contester devant le tribunal judiciaire durant un délai de six mois” — the reasoned decision is notified to the declarant, who may challenge it before the civil court within six months. Declarations on grounds of marriage are received by the administrative authority and a receipt is issued once the documents proving admissibility are supplied (article 26 of the Civil Code). Silence for a year after the receipt therefore works in your favour: without a refusal inside the deadline, a copy of the declaration is returned to you bearing the registration stamp, and you are French. What follows explains each blocking route separately, because clients constantly confuse them and the confusion costs files. A refusal to register says your file did not meet the legal conditions — the marriage is too recent, the shared life is doubted, the French is too weak, a document is missing — and it is made by the minister or the court registrar and challenged before the tribunal judiciaire (the ordinary civil court). An opposition says the conditions may be met but the Government does not want you as a citizen — unworthiness or failed assimilation — and it is made by decree after advice of the Conseil d’État and challenged before the administrative supreme court. Different authors, different judges, different arguments. Identify which one hit you before doing anything else.

A. Challenging a refusal to register within six months before the civil court

A refusal must be reasoned, notified to you, and issued within one year of the receipt that closed your file at the interview. The six-month challenge period runs from notification, and it is strict: a late claim is inadmissible however strong the merits. The action is brought by writ (assignation) against the public prosecutor, before the tribunal judiciaire for your home area, and one formality traps the unwary — a copy of the writ must be lodged with the Ministry of Justice, which issues its own receipt. A recent Paris nationality judgment shows judges checking this first: the tribunal judiciaire of Paris, 20 June 2025, RG 22/02397, opened by confirming that “le ministère de la justice a délivré ce récépissé le 12 juillet 2022” and that the procedural condition was therefore satisfied, before turning to the substance of a marriage-based declaration signed in 2021. Substance is where British files usually live or die, because the burden of proof rests on you. The same judgment recalls that the declarant must prove “d’une part, d’un état civil fiable et certain, et, d’autre part, … que les conditions de la déclaration de nationalité française posées par l’article 21-2 du code civil sont remplies” — first, a reliable and certain civil status, and second, that the conditions of article 21-2 are fulfilled. Civil status means original full birth and marriage certificates, and foreign records are read through article 47 of the Civil Code: an act drawn up abroad in local form is trusted unless other documents, outside information or the act itself shows it is irregular, forged or untrue. In that Paris case the court was not convinced, and the operative part is a warning worth quoting in full: “Juge que M. [K] [U] [C] [A] … n’est pas de nationalité française”, with the marginal note provided for by article 28 ordered on the relevant records and costs against the claimant. Losing does not always end the story — a fresh declaration remains possible once the missing condition is genuinely fixed — but the judgment, the note in the margin of your certificates and the wasted years make a first badly prepared attempt an expensive mistake. Build the winning file before you sue: recent full certificates, the transcription proving four years of marriage, three years of residence permits and bills or the consular registration certificate of your French spouse, a B2 certificate less than two years old, a clean foreign criminal record, and a coherent story of shared life told through leases, joint tax assessments, children’s school records and photographs of nothing in particular — ordinary life, documented. Where the refusal attacks your French, re-sit the TCF or TEF before suing and exhibit the new pass certificate; where it attacks the shared life, gather the paper trail of a real household rather than protestations. And watch the calendar from both ends: the administration’s one-year deadline from the receipt, which a refusal must respect, and your six months from notification, which you must respect. Either deadline missed by its author changes everything — an out-of-time refusal is unlawful, and an out-of-time challenge is dead.

B. Defeating a State opposition: unworthiness, failed assimilation and review by the supreme administrative court

The opposition is the heavier weapon. Article 21-4 of the Civil Code provides that “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” — the Government may oppose, by decree in Conseil d’État, on grounds of unworthiness or failed assimilation other than linguistic, within two years from the receipt, or from the day the court judgment admitting a refused declaration becomes final. Language cannot be the ground here, precisely because it was already tested at registration; the Government must point to conduct. The statute names two automatic cases of failed assimilation: an actual situation of polygamy, and a conviction for the most serious sexual offences against a child under fifteen. Beyond those, indignité (unworthiness) covers serious criminal offending, and défaut d’assimilation (failed assimilation) covers behaviour showing rejection of the Republic’s essential principles or deliberate withdrawal from the national community — the mirror of the interview test described above. If the decree is made, its effect is brutal: you are deemed never to have become French, though acts you performed between the declaration and the decree cannot be attacked merely for want of nationality. But a decree is not the last word. It is an administrative decision reviewable for excès de pouvoir (excess of power, the action for annulment of an unlawful administrative act) directly before the Conseil d’État, and the judges re-examine whether the file really showed what the Government claimed. The freshest illustration is Conseil d’État, 29 May 2026, no. 501856, where a declarant challenged a decree opposing her acquisition on failed-assimilation grounds. The Court held: “Il ne ressort pas des éléments versés au dossier par le ministre de l’intérieur, en particulier du rapport de synthèse … des services de la préfecture … et du compte-rendu d’entretien complémentaire …, que Mme A… adopte un comportement ou soutient des thèses manifestant un rejet des principes essentiels de la République ou se tient délibérément à l’écart de la communauté nationale” — nothing in the minister’s evidence, neither the prefecture’s summary report nor the further interview record, showed conduct or views rejecting the Republic’s essential principles or deliberate withdrawal from the national community. Consequently “Mme A… est fondée à demander l’annulation pour excès de pouvoir du décret qu’elle attaque”, and the decree was annulled. Three lessons for British spouses follow. First, the Government must prove its case from the file — vague assertions about integration do not survive review, and prefecture reports plus interview records are dissected line by line. Second, your conduct at the interviews and in daily administrative life is evidence: consistent statements, visible participation in local and family life, and respect for the values questions all end up quoted in the judgment. Third, the standard is rejection of essential principles or deliberate withdrawal, not imperfect Frenchness — an accent, British habits, a house full of Marmite and Match of the Day are not failed assimilation, and no prefecture report should be allowed to dress them up as such. Time limits matter here too: act quickly once the decree is notified, instruct counsel experienced in nationality litigation, and obtain the full administrative file — the prefecture report, the interview records, the documents the minister relied on — because the annulment in 2026 was won by holding each of those papers up to the light. Even after registration without opposition, remember the prosecutor’s long arm: registration may still be contested within two years if the legal conditions were not met, and within two years of discovery in case of lies or fraud, with separation inside twelve months raising the presumption of fraud. Nationality by marriage is a right when the conditions hold, but it is a watched right for years afterwards.

Conclusion

For a British spouse, the marriage declaration remains the most direct road to a French passport: four years of marriage, a shared life that is real on declaration day, transcribed civil status, lawful residence or a properly registered French spouse abroad, B2 French, and a clean file — and the declaration must then be registered, with nationality backdated to your signature. The two traps are equally clear. A refusal to register must come reasoned and within a year of your receipt, and you have six months to answer it before the civil court, where you carry the burden of proving every condition. A Government opposition must come by decree within two years on unworthiness or non-linguistic failed assimilation, and it answers to the Conseil d’État, which annuls decrees the file does not support, as it did in May 2026. In both battles the evidence that counts is gathered long before the fight: the permits kept current, the bills in both names, the transcription requested early, the B2 certificate earned in time, and interviews prepared as the examinations they are. Get those right and the declaration procedure is what the statute promises — not a favour to be begged for, but a right to be registered.