Your London employer wants you in Lyon for eight months, on your British contract and your British salary. Since Brexit, that familiar arrangement — the posting, known in French as détachement — sits at the crossing of three systems that no longer talk to each other automatically: British social security, French labour law and the France-UK double tax treaty. Get the paperwork right and you keep paying National Insurance at home, you work under a clear French floor of pay and safety rights, and your salary stays taxable in only one country. Get it wrong and the French social security collection agency, URSSAF (the body that collects social contributions), can bill your employer for full French contributions, the labour inspectorate can fine the operation thousands of euros per worker, and both countries can claim tax on the same pay.
This guide is written for the British employee who is sent to work in France while remaining employed by a UK business — whether for a client contract in Paris, an intra-group mission in Toulouse or a factory installation in Lille. It is not about moving to France permanently, working remotely from France for a UK employer, or signing a French local contract. Those are different situations with different rules. Here, the question is narrower and more urgent: how a temporary British posting survives a French inspection, and what to do when it does not.
The first part explains your status: how you lawfully keep British social security through the A1 certificate under the UK-EU Trade and Cooperation Agreement, which French declarations must be filed before you start, and where your salary is taxed. The second part explains your daily rights on French soil — minimum pay, working time, health and safety — and the controls you may face, with the exact routes for challenging a fine, a back-payment bill or a finding of fraud. Every decisive statement below is tied to the official text or decision it comes from, quoted word for word.
I. Posting, not moving: your status during a British secondment to France
A posting is a triangular situation. You keep a contract of employment with your UK employer, that relationship continues throughout the mission, and you work temporarily on French territory. French law defines it exactly that way: an employer established outside France may post employees temporarily provided a contract of employment exists and the employment relationship continues during the posting, whether for a client contract, inside the same group, or on the employer’s own account without any French client at all. That definition, in Article L1262-1 of the French Labour Code, is the doorway to everything that follows. If your situation does not fit through it — if, for example, you were recruited in London specifically to work permanently in France — you are not posted at all, and French affiliation and a French contract apply from day one.
A. Keeping British social security: the 24-month posting, the A1 form and the SIPSI declaration
The starting principle is simple and strict. A worker who moves inside Europe is subject to one social security system only, normally the system of the country where the work is done. That single-system principle was preserved between the United Kingdom and the European Union, including France, by the Trade and Cooperation Agreement. Posting — détachement — is the exception that lets you stay in the British system while working temporarily in France. The French liaison body for international social security, CLEISS (Centre des liaisons européennes et internationales de sécurité sociale), states the deal plainly: posting allows your continued cover under the British social security regime during a temporary mission on French territory, up to a maximum of 24 months. Two conditions must both be met: you must not be sent to replace another worker, and the planned mission must not exceed 24 months. Then comes the rule that has ended many optimistic secondment plans: no extension and no exceptional derogation are possible, as CLEISS confirms for United Kingdom to France postings. A posting that runs past 24 months, or one worker succeeding another on the same French post beyond the limit, falls out of the British system and into French affiliation, with French rates of contributions.
If your mission began before 2021, different rules apply: postings already running when Brexit took effect remain governed by the pre-Brexit European rules under the Withdrawal Agreement, not by the Trade and Cooperation Agreement. Check the start date of the mission first, because it decides which legal regime you live under.
In practice, the posting is proved by a single document: the A1 certificate. Your UK employer must apply to the competent British institution — in practice HMRC — and if the conditions are satisfied, the British institution issues an A1 form certifying that you remain under British social security during the mission in France. CLEISS explains the effect: during the certificate’s period of validity, social contributions are due in the United Kingdom, not in France, and you must keep the form with you throughout the work in France and be able to show it to the French authorities at any inspection. If the British institution refuses, contributions for your work are obligatorily due in France at French rates. On the British side, HMRC runs the application through form CA3822, the certificate confirming you pay UK National Insurance while working temporarily abroad, which employers, employees or their agents can apply for online — though not more than 12 months in advance. Apply early: an A1 requested after the inspector has already knocked carries far less weight than one obtained before the mission began.
The A1 is only half of the file. French labour law imposes its own prior formalities on the posting employer, and they must be completed before the posting starts. The employer must send a prior declaration of posting — déclaration préalable de détachement — to the local labour authority (the DDETS, the departmental directorate for employment, labour and solidarities) of the place where the work begins, through the official SIPSI portal (the national online system for posting declarations). Article L1262-2-1 of the Labour Code states the duty in these terms: the employer “adresse une déclaration, préalablement au détachement, à l’inspection du travail du lieu où débute la prestation” — sends a declaration, before the posting, to the labour inspectorate of the place where the service starts. The same article requires the employer to appoint a representative in France to liaise with the inspection services for the duration of the work, and it warns that completing these formalities does not presume that the posting itself is lawful. The declaration and the representative are procedural duties; the substance — a genuine temporary posting — must also exist.
Two further duties follow you onto the French site. First, translation: Article L1263-7 requires the posting employer or its French representative to hold available at the place of work, for the labour inspectorate, documents translated into French proving compliance with the posting rules — “présente sur le lieu de réalisation de la prestation à l’inspection du travail des documents traduits en langue française”. An English-only file shown to a French inspector is a breach in itself. Second, immigration: since Brexit, British nationals are third-country nationals in France, and a posted worker generally needs a work authorisation — autorisation de travail — applied for online before the mission. The official service-public guidance confirms that where a company not established in France temporarily posts an employee, the work authorisation application is made by the French ordering party (the client) or, for agency postings, by the French user company. The online filing receipt alone does not authorise work. One exception matters for group transfers: the long-stay visa or residence card marked “ICT posted employee” (salarié détaché ICT, for intra-corporate transferees) itself authorises the posted work, and the employer is then dispensed from the separate work permit application. Identify before departure which route you travel under, because the wrong assumption surfaces at the first document check.
Fail any of these elements — no A1, no SIPSI declaration, no representative, no translated documents, no work authorisation — and the consequences stack. Without a valid A1, French affiliation applies and URSSAF can assess full French employer and employee contributions on the salaries paid during the mission. Missing declarations and papers draw administrative fines, examined in Part II below. And where the posting is a sham — a letter-box arrangement with no genuine UK activity — the criminal courts can reclassify the operation as concealed work, travail dissimulé, with the A1 set aside as fraudulent. The lesson of this section is therefore practical: assemble the complete file before boarding the Eurostar — A1 applied for and ideally issued, SIPSI declaration filed, French representative designated, translated documents ready, work authorisation granted — and calendar the 24-month limit from the first day, because no prolongation exists under the Trade and Cooperation Agreement.
B. Paying tax in the right country: the 183-day rule and the French return
Social security and tax follow different maps, and posted workers constantly confuse them. The A1 keeps you in British social security; it does not decide where your salary is taxed. That question belongs to the France-UK double tax treaty signed on 19 June 2008, and specifically to its employment article — Article 15 on income from employment in the official treaty text. The architecture has two floors. First, a salary is taxable only in your country of residence unless the work is done in the other country: pay stays taxable only in the residence country unless the employment is exercised in the other country, in which case the pay derived from that work may be taxed there. So the days you physically work in France may be taxed in France. Second, there is an escape back to residence-only taxation, and it has three cumulative conditions: the worker is present in the other country for no more than 183 days in aggregate in any 12-month period, the pay is met by an employer not resident in that other country, and the pay is not borne by a permanent establishment the employer has there. Miss any one of the three and France may tax the French-worked days.
Apply this to the standard British posting. You remain UK resident, you spend fewer than 183 days in France in any 12-month window, your UK employer has no French branch bearing your salary: all three conditions hold, and your salary stays taxable only in the United Kingdom. HMRC taxes it through PAYE as usual, and France has no claim. Cross any of the lines and the outcome changes. Stay 183 days or more and France may tax the portion of pay attributable to French working days, with the United Kingdom then relieving the double tax under the treaty’s elimination rules. Be paid through, or have your cost recharged to, a French permanent establishment of your employer and the second condition fails. Become French tax resident — by moving your main home to France, spending most of the year there, or shifting your centre of economic interests — and the whole analysis restarts with France as the residence state: worldwide salary declared in France on forms 2042 and 2047, with a treaty credit for any UK tax on the same income. Residence under domestic law and the treaty’s tie-breaker are examined in detail in the companion guide to your first French tax return after moving from the UK; what matters here is not to assume that a short posting can never create French tax residence. A posting that stretches past a year, with family and home relocated, often does.
Two satellite points complete the picture. First, social contributions follow affiliation, not the treaty: with a valid A1 you pay UK National Insurance and no French social charges on the salary; without one, French contributions apply at French rates, and the CSG-CRDS position follows the same affiliation logic. Second, keep evidence of presence: the 183-day count is proved day by day — travel records, boarding passes, assignment letters, timesheets. When the French tax administration challenges the treaty position, the file that wins is the one that counts days. File the French return whenever French-source pay exists or residence has shifted, claim the treaty position explicitly, and challenge any double charge through a formal claim — réclamation — within the statutory time limit, appealing to the administrative court if refused.
II. Working on French soil: the rights France guarantees and the controls you may face
Once lawfully posted, you work under two layers at once: your British contract continues, and a French floor of employment rights applies beneath it. France then polices the arrangement through two separate inspection channels — the labour inspectorate (inspection du travail) for working conditions and declarations, and URSSAF for social contributions — each with its own sanctions and each with its own routes of challenge.
A. The French floor under your British contract: pay, hours and safety
France does not rewrite your British contract, but it guarantees you equal treatment with employees of French companies in the same sector on a defined list of subjects — the “hard core” of posting law. Article L1262-4 of the Labour Code requires the posting employer to ensure compliance with the legal and collectively agreed rules applicable to French-sector employees on eleven matters, including individual and collective freedoms at work, non-discrimination and equality between women and men, maternity and parental leave, working time, compensatory rest, public holidays and paid annual leave, submission to leave funds, pay within the meaning of equal-pay rules including overtime uplifts, health and safety at work, illegal work, and the reimbursement of posting-related travel, meal and accommodation expenses. In everyday terms: the French minimum wage (SMIC) and any higher sector minimum, the 35-hour reference and overtime premiums, paid holidays, and French safety rules apply to your French working days even though your contract remains British. Your employer must also reimburse the extra costs the mission imposes — transport, meals, lodging — where they are inherent to the posting.
Time extends the floor. If the posting exceeds twelve months, then from the thirteenth month the employer becomes subject to nearly all of the Labour Code provisions applicable to French-established companies, with only narrow exceptions. Successive workers replacing one another on the same post count together toward the twelve months. A posting planned for fourteen months therefore lives under two regimes: the hard core first, then almost the full Code. Price that into the assignment letter before it starts.
Two court rulings frame what the A1 does and does not do to these rights, and both help the posted worker. First, a valid foreign certificate never lowers the French floor. The Court of Cassation, following the Court of Justice of the European Union, holds that an E101 or A1 certificate binds the courts of the host country only in social security matters — “s’imposent aux juridictions de ce dernier État membre uniquement en matière de sécurité sociale” — and, crucially, that “le maintien d’un certificat E 101 ne fait pas obstacle à ce que le juge de l’Etat membre d’accueil applique les règles nationales de droit du travail relatives à cette relation de travail et sanctionne ainsi la violation par l’employeur d’obligations que le droit du travail met à la charge de celui-ci” (Cass. Soc., 31 March 2021, No. 16-16.713). In plain English: keeping the certificate does not prevent the French judge from applying French labour law to the relationship and punishing the employer’s breaches. Unpaid French overtime, missing leave, sub-SMIC pay — the A1 is no shield against any of them, and you can claim the difference before the French employment tribunal (conseil de prud’hommes).
Second, a defective posting does not make the French client your employer. Where the foreign employer has breached the posting rules, the worker sometimes argues that the French host company must be treated as the real employer, with a French contract and its protections. The Court of Cassation refused that shortcut: “le non-respect, par l’employeur étranger, des règles relatives au détachement, sur le territoire national, d’un de ses salariés, n’a pas pour effet de voir reconnaître la qualité d’employeur à l’entreprise établie sur le territoire national et bénéficiaire dudit détachement” — the foreign employer’s breach of the posting rules does not result in the French beneficiary company being recognised as the employer (Cass. Soc., 5 April 2023, No. 21-21.318). Your claims for French minimum rights therefore run against your UK employer, not against the French client whose site you work on — while the client faces its own separate fines for using an irregular posting, examined below. Sue the right defendant.
B. Inspections, fines and back-tax bills: how to challenge them
French controls arrive through two doors, and you must know which one has been knocked. The labour inspectorate checks the posting file: the SIPSI declaration, the French representative, the translated documents held on site, the work authorisation, and the substantive working conditions. URSSAF checks the money: whether contributions were due in France and paid. Each can act on the same mission, and each decision is challenged differently.
Start with URSSAF, because its powers were widened by a very recent ruling every posted worker should know. On 16 October 2025, the Court of Cassation held that the social security bodies’ approved and sworn inspection agents, when investigating concealed work involving employees working in France for an employer headquartered abroad, verify the declarations the employer owes to the French protection and collection bodies by collecting from the employer the documents, whatever their form or medium, that show whether the employer is affiliated to French social security: “vérifient les déclarations qui doivent être faites par l’employeur aux organismes de protection sociale et de recouvrement des contributions et cotisations sociales en vertu des dispositions légales en vigueur en recueillant auprès de l’employeur les documents, quels que soient leur forme et leur support, qui permettent d’établir si l’employeur dispose d’une affiliation à la sécurité sociale française” (Cass. Civ. 2nd, 16 October 2025, No. 23-14.039). The appeal court had annulled the reassessment, reasoning that URSSAF agents could only demand the narrow set of anti-illegal-work documents; the Court of Cassation quashed that decision entirely — “CASSE ET ANNULE, en toutes ses dispositions, l’arrêt” — and sent the case to Metz. The message for British postings is direct: produce the A1 and the affiliation file at the control, because URSSAF is entitled to demand every document that proves where the employer is affiliated, and a missing A1 at that moment is the fastest route to a full French reassessment — redressement — covering the whole mission.
Where the A1 exists but looks suspicious, France cannot simply tear it up. A 2026 criminal ruling restates the European fraud procedure that protects genuine certificates and organises the attack on false ones. The Court of Cassation recalls that a court of the host country may set aside A1 certificates found to be fraudulent only after a mandatory dialogue-and-conciliation procedure with the issuing institution has been promptly launched and the issuer has failed to genuinely re-examine the certificates in light of the concrete evidence submitted — the Altun and Vueling case law of the Court of Justice (Ömer Altun, 6 February 2018, C-359/16; Vueling Airlines, 2 April 2020, C-370/17 and C-37/18). Quoting the Vueling requirements, the Court insists that judges may find fraud and set the certificates aside only after verifying “que la procédure de dialogue et de conciliation a été promptement enclenchée” and “que l’institution compétente de l’État membre d’émission s’est abstenue de procéder à un tel réexamen” — that the dialogue procedure was promptly triggered and that the issuing institution failed to carry out such a re-examination (Cass. Crim., 9 June 2026, No. 24-85.090). For the British worker on a genuine mission, this is a shield: an A1 properly issued by HMRC can only be discarded through that adversarial procedure, and any reassessment that ignores it is vulnerable on appeal. For sham arrangements — no real UK activity, workers permanently in France under cover of rolling certificates — it is a warning: once the dialogue procedure documents the fraud, the certificates fall and French contributions, back taxes and criminal liability for concealed work follow.
The labour inspectorate’s sanctions are administrative fines, and their scale concentrates minds. Breaches by the posting employer — missing SIPSI declaration, no French representative, missing documents — are punishable under Article L1264-1, while the French client or user company that uses an irregular posting faces its own fines under Article L1264-2, notably for failing its vigilance duties toward its contractor. Article L1264-3 sets the tariff: “Le montant de l’amende est d’au plus 4 000 € par salarié détaché et d’au plus 8 000 € en cas de réitération dans un délai de deux ans à compter du jour de la notification de la première amende. Le montant total de l’amende ne peut être supérieur à 500 000 €” — up to 4,000 euros per posted worker, 8,000 on a repeat within two years, capped at 500,000 euros, calibrated to the circumstances, gravity, good faith, resources and liabilities of the offender, with a two-year limitation period. The same article gives the route of challenge: the employer, client or ordering party “peut contester la décision de l’administration devant le tribunal administratif” — may challenge the administration’s decision before the administrative court, to the exclusion of any hierarchical appeal. The two-year clock, the good-faith discount and the exclusivity of the judicial route are all arguments to plead, with evidence, from the first response to the statement of objections.
Put together, the challenge playbook for a British posted worker — and for the UK employer behind the posting — runs as follows. At the inspection, cooperate through the French representative, produce the A1, the SIPSI receipt, the translated documents and the work authorisation, and record exactly what is requested and seized. Against a URSSAF reassessment, contest through the standard social-security dispute route: an administrative appeal before the amicable appeals commission, then the judicial division of the judicial court (pôle social), invoking the A1’s binding effect in social security matters and, where the A1 is attacked, the absence of a proper dialogue procedure. Against a labour-administration fine, go directly to the administrative court within the time limit stated on the decision, pleading good faith, first offence, regularisation and proportionality. Against a French tax charge on the salary, file the treaty-based claim and appeal a refusal to the administrative court. And for unpaid French minimum rights — overtime, leave, SMIC top-ups — bring the claim against the UK employer before the employment tribunal, armed with the payslips, timesheets and assignment letters the French file already contains. The companion guide to the posting formalities seen from the employer’s side completes the picture for the company sending you. In every route, the same rule decides the outcome: the party with the contemporaneous paper — A1, declaration, day counts, pay records — wins.
Conclusion
A British posting to France survives on three papers and one calendar. The A1 certificate keeps you in British social security for up to 24 months with no prolongation possible; the SIPSI declaration, the French representative, the translated documents and the work authorisation keep you lawful in the eyes of the labour inspectorate; and the treaty’s 183-day triple test keeps your salary taxable in one country only. Beneath your British contract, France lays a floor you can enforce — minimum pay, working time, leave, safety — that no A1 can waive, and it polices the arrangement with inspectors whose powers the Court of Cassation has just confirmed, fines of up to 4,000 euros per worker, and reassessments that can reprice an entire mission. The defences are real but procedural: a genuine A1 can only be set aside through the European dialogue procedure, fines are challenged before the administrative court with good faith and proportionality arguments, and pay claims run against the UK employer before the employment tribunal. Assemble the file before departure, count the days and the months as the mission runs, and challenge quickly and in the right court when a decision lands. A posting prepared that way is not a risk to fear but a framework to use.