Your London employer wants you in Paris for eight months to lead a project, install equipment, or rescue a struggling subsidiary. Before Brexit, this was a formality: you boarded the Eurostar, kept paying UK National Insurance, and came home. Today, you are a third-country national going to work in France, and two administrations will each ask you for paperwork before you start: the French labour inspectorate, which wants your posting declared on a system called SIPSI, and the social security liaison bodies, which want proof of which country covers you. Get either file wrong and the bill lands fast: administrative fines of up to 4,000 euros per posted worker, a criminal file for concealed employment, a refusal or withdrawal of your residence card, or a URSSAF reassessment with interest and surcharges.

This guide is written for the posted person, the British employee sent temporarily to France by a UK employer that keeps your employment contract alive. It answers the two questions every secondee asks first: can I still be posted to France after Brexit and what must be filed before day one, and which country’s social security covers me while I am there. It then sets out the remedies: how to challenge an ICT card refusal, what happens when a residence permit expires, and how to fight a URSSAF reassessment. Every decisive statement below rests on a statutory text or a court decision quoted word for word from the official source, with the link beside it.

I. Can a British employee still be posted to France after Brexit, and what must be filed before day one?

A. Do I need a French visa or work permit for a secondment from the UK?

Start with the vocabulary, because the French system uses one word, détachement, for three different situations. A détachement, usually translated as posting or secondment, means your UK employer sends you to work temporarily on French soil while your British employment contract continues. Article L. 1262-1 of the French Labour Code states the rule in these terms: “Un employeur établi hors de France peut détacher temporairement des salariés sur le territoire national, à condition qu’il existe un contrat de travail entre cet employeur et le salarié et que leur relation de travail subsiste pendant la période de détachement.” In plain English: a foreign employer may post workers to France only while a genuine employment relationship with that employer survives throughout the assignment. If your UK contract is terminated and replaced by a French contract, you are no longer posted; you are locally hired, and a different residence regime applies.

The same article lists three posting patterns. The first covers service contracts: you work in France for a French client under a contract between your UK employer and that client. The second covers intra-group mobility: you are sent to a French establishment or company of your own group. The third covers postings with no French client at all, for example an engineer sent to commission the group’s own new plant. Your pattern matters because it decides which residence document you need.

For short trips, remember the Schengen ceiling. As a British citizen you may spend up to 90 days in any 180-day period in the Schengen area without a visa, but visa-free presence is not permission to work. A short posting to perform actual work in France needs a work authorisation or the appropriate visa even inside the 90 days, unless a specific exemption covers your activity. Never assume that a stamp-free entry at Gare du Nord authorises you to start on site on Monday morning.

For assignments of several months inside your own group, the dedicated route is the multi-year residence card marked “salarié détaché ICT”, named after the EU intra-corporate transfer system. Article L. 421-26 of the Code on the Entry and Stay of Foreigners (CESEDA) provides: “L’étranger résidant hors de l’Union européenne qui vient en France pour effectuer un détachement temporaire intragroupe, prévu au 2° de l’article L. 1262-1 du code du travail, afin d’occuper un poste d’encadrement supérieur ou d’apporter une expertise dans un établissement ou une entreprise du groupe qui l’emploie, et qui justifie d’une ancienneté professionnelle d’au moins six mois au sein de ce groupe, se voit délivrer une carte de séjour pluriannuelle portant la mention ” salarié détaché ICT ” valable pour la durée du détachement temporaire, dans la limite de trois ans.” Three practical filters emerge from that sentence. First, you must be a manager or an expert, not general staff. Second, you need at least six months of service within the group before the move. Third, the card lasts for the assignment up to three years, it cannot be renewed, and a fresh card requires six months spent outside the European Union first. For missions lasting more than three months but no more than one year, the official service-public.fr page on the ICT posted employee card explains that a long-stay visa valid as a residence permit (VLS-TS) with the ICT mention is used instead of the card. Your spouse and children are not forgotten: the family members of a mobile ICT worker receive their own multi-year card, as CESEDA Article L. 421-29 organises for ICT families.

The refusal grounds are published on that same official page and deserve reading before you apply. The prefecture, the local state authority that issues residence cards, must refuse the ICT card where your employer or the host company has been convicted of fraud to obtain a foreign worker’s permit, or where the three-year maximum stay has been reached. It may refuse where the employer has been convicted of employing foreigners without authorisation. A refusal is never the end of the road; the remedies are set out in part II below.

B. What must my UK employer declare on SIPSI, and what happens if it files nothing?

While you deal with the prefecture, your employer deals with the labour inspectorate, known as the inspection du travail. Every posting must be declared in advance on SIPSI, the online prior-declaration portal run by the Ministry of Labour. The Ministry states the duty bluntly on its SIPSI page: Every employer established outside France that posts workers onto French soil must send the labour inspectorate of the place of work a prior posting declaration before its operation in France begins. No declaration, no legal start: the filing must reach the inspectorate of the place where you will actually work before the assignment begins.

The statute behind that page is Article L. 1262-2-1 of the Labour Code. Its first paragraph requires the posting employer to send “une déclaration, préalablement au détachement, à l’inspection du travail du lieu où débute la prestation”. Its second paragraph adds a concrete obligation many British companies miss: the employer “désigne un représentant de l’entreprise sur le territoire national, chargé d’assurer la liaison avec les agents mentionnés à l’article L. 8271-1-2 pendant la durée de la prestation”. You need a named contact person in France available to the inspectors for the whole assignment. A third paragraph warns against false comfort: “L’accomplissement des obligations mentionnées aux I et II du présent article ne présume pas du caractère régulier du détachement.” Filing the declaration does not prove the posting is genuine; inspectors look behind the paperwork at the reality of the employment link.

During the assignment, the employer must also keep French-language documents on site. Article L. 1263-7 requires the posting employer or its representative to show the inspectorate, at the place of work, “des documents traduits en langue française permettant de vérifier le respect des dispositions du présent titre”. Payslips, time records and the A1 social security certificate should be available in French on demand.

The substantive rights you carry with you are called the “hard core”, the noyau dur. Article L. 1262-4 guarantees you equality of treatment with employees of same-sector companies established in France: “L’employeur détachant temporairement un salarié sur le territoire national lui garantit l’égalité de traitement avec les salariés employés par les entreprises de la même branche d’activité établies sur le territoire national”. In practice that means French rules on minimum pay, working time, rest, health and safety and equal treatment follow you onto the French site, even though your contract stays British.

Non-compliance is priced, per worker. Article L. 1264-1 makes breach of the declaration, representative and document duties punishable by administrative fine, and Article L. 1264-3 sets the scale: “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 €.” The authority weighs the circumstances, the gravity, the employer’s behaviour and good faith. Five undeclared engineers on one Paris site can therefore cost up to 20,000 euros on a first finding, before any criminal exposure.

The criminal exposure is real and recent. Concealed employment, travail dissimulé, is defined by Article L. 8221-5: “Est réputé travail dissimulé par dissimulation d’emploi salarié le fait pour tout employeur : 1° Soit de se soustraire intentionnellement à l’accomplissement de la formalité prévue à l’article L. 1221-10 , relatif à la déclaration préalable à l’embauche ; 2° Soit de se soustraire intentionnellement à la délivrance d’un bulletin de paie ou d’un document équivalent défini par voie réglementaire, ou de mentionner sur le bulletin de paie ou le document équivalent un nombre d’heures de travail inférieur à celui réellement accompli, si cette mention ne résulte pas d’une convention ou d’un accord collectif d’aménagement du temps de travail conclu en application du titre II du livre Ier de la troisième partie ; 3° Soit de se soustraire intentionnellement aux déclarations relatives aux salaires ou aux cotisations sociales assises sur ceux-ci auprès des organismes de recouvrement des contributions et cotisations sociales ou de l’administration fiscale en vertu des dispositions légales.” A posting with no SIPSI declaration, no French payroll filings and no contributions is the textbook illustration of the third limb.

The courts apply this to foreign operators without hesitation. On 2 March 2021 the Criminal Chamber upheld convictions against the managers of a Slovak temporary-work agency operating in France (Cass. crim., 2 March 2021, No. 19-80.991). And on 9 June 2026 the same Chamber ruled in a major organised posting-fraud case involving Spanish certificates, with a 500,000-euro fine against the company, four-year suspended prison terms for the individuals, confiscations and permanent professional bans. On the scope of the professional ban, the Court held (Cass. crim., 9 June 2026, No. 24-85.090): “En prononçant une peine d’interdiction définitive de gérer toute entreprise ou toute société, alors que les articles 131-27 du code pénal et L. 8224-3 du code du travail, applicables au délit de travail dissimulé, limitent une telle interdiction aux entreprises commerciales ou industrielles et aux sociétés commerciales, la cour d’appel a méconnu le texte susvisé et le principe ci-dessus rappelé.” Even in a fraud case, penalties must stay inside their statutory frame, which is also your protection if an over-broad sanction ever hits your employer.

One last warning in this section concerns your French client. If you are posted under a service contract, the French customer has a vigilance duty. On 21 February 2023 the Criminal Chamber recalled, under Articles L. 8221-1 and D. 8222-7 (Cass. crim., 21 February 2023, No. 22-81.903): “Il résulte du premier de ces textes qu’est interdit le fait de recourir sciemment, directement ou par personne interposée, aux services de celui qui exerce un travail dissimulé.” And: “Il se déduit du second que la personne morale qui contracte avec une entreprise établie ou domiciliée dans un autre Etat membre de l’Union européenne doit, dans tous les cas, se faire remettre par celle-ci le certificat A1 attestant de la régularité de la situation sociale du cocontractant au regard du règlement (CE) n° 883/2004 du Parlement européen et du Conseil du 29 avril 2004 portant coordination des systèmes de sécurité sociale pour chacun des travailleurs détachés auxquels elle a recours.” Ask your French client for its checklist early: it is legally required to demand your A1 certificate, and its failure helps no one.

II. Which country’s social security covers me while posted, and who pays if things go wrong?

A. Do I stay on UK National Insurance or join the French system while posted?

The starting point in France is affiliation by territory. Article L. 111-2-2 of the Social Security Code provides: “Sous réserve des traités et accords internationaux régulièrement ratifiés ou approuvés et des règlements européens, sont affiliées à un régime obligatoire de sécurité sociale dans le cadre du présent code, quel que soit leur lieu de résidence, toutes les personnes : 1° Qui exercent sur le territoire français : a) Une activité pour le compte d’un ou de plusieurs employeurs, ayant ou non un établissement en France”. Read alone, that sentence would put every posted Briton into the French system from day one. The six words that change everything are the first ones: “sous réserve des traités et accords internationaux”, subject to international treaties and agreements.

For the United Kingdom, the applicable agreement is the EU-UK Trade and Cooperation Agreement and its Protocol on Social Security Coordination. The French liaison body, the Cleiss, explains on its official Brexit FAQ: The cooperation agreement in force since 1 January 2021 keeps postings between the United Kingdom and France possible, with postings strictly capped at 24 months, the other main rules including prior notification maintained, and the portable A1 document required for assignments starting after that date. In practice, your UK employer applies to HMRC before departure for an A1 portable document, and while that A1 runs, you keep paying UK National Insurance instead of French contributions, for up to 24 months. The British mirror is the GOV.UK guidance on National Insurance abroad: you need a certificate of coverage proving you pay National Insurance in the UK so that no social security contributions are due in the country where you work. Apply before you travel: an A1 requested after the inspector knocks carries far less weight.

The French courts police the boundary between genuine postings and artificial ones. On 20 March 2025 the Second Civil Chamber quashed a decision that had annulled a URSSAF reassessment, recalling first the territorial principle (Cass. 2nd civ., 20 March 2025, No. 23-12.264): “Selon le premier de ces textes, toute personne exerçant une activité salariée ou non sur le territoire français est obligatoirement affiliée au régime obligatoire de sécurité sociale français, sous réserve des traités et accords internationaux.” The Court then held (Cass. 2nd civ., 20 March 2025, No. 23-12.264) that a local mission contract could not prove a posting where the lower court gave “motifs impropres à caractériser que M. [R] était détaché de la société [4] avec maintien d’un lien contractuel et de subordination avec cette dernière”. The lesson is direct: without a surviving contract and a real chain of subordination to the UK employer, there is no posting, and French affiliation plus reassessment follow.

Understand also what the A1 does and does not prove. In the Slovak agency case cited above, the Criminal Chamber (Cass. crim., 2 March 2021, No. 19-80.991) recalled the Court of Justice’s Bouygues ruling of 14 May 2020 (C-17/19): “les formulaires de détachement, dits certificats E 101 et A1, s’imposent aux juridictions de l’Etat sur le territoire duquel les travailleurs exercent leurs activités uniquement en matière de sécurité sociale.” The same decision explains that these certificates bind the host State only as proof of social security affiliation and produce no binding effect on obligations imposed by national law in other fields. An A1 settles social security affiliation; it does not legalise the posting under labour law, immigration law or criminal law. A valid A1 never excuses a missing SIPSI declaration.

Conversely, a fraudulent A1 can be swept aside, but only after a mandatory dialogue between the two countries’ institutions. In its 9 June 2026 decision (Cass. crim., 9 June 2026, No. 24-85.090) the Criminal Chamber stated: “La mise en oeuvre de la procédure de dialogue et de conciliation prévue au règlement (CE) n° 883/2004 du Parlement européen et du Conseil du 29 avril 2004 sur la coordination des systèmes de sécurité sociale, qui constitue un préalable obligatoire à un éventuel constat définitif de fraude par les autorités compétentes de l’Etat d’accueil, lorsque les poursuites pour travail dissimulé ont été engagées pour défaut de déclarations aux organismes de protection sociale (CJUE, arrêt du 4 mai 2020, Bouygues travaux publics e.a., C-17/19)”. The same ruling adds (Cass. crim., 9 June 2026, No. 24-85.090): “le fait pour l’institution compétente de l’Etat membre d’émission de confirmer le bien-fondé de certificats A1 sans se prononcer sur les éléments concrets soumis par l’institution compétente de l’État membre d’accueil qui donnent à penser que ces certificats ont été obtenus ou invoqués de manière frauduleuse ne peut être assimilé à un réexamen effectif. Dans ce cas, et dès lors qu’elle constate le caractère frauduleux de ces certificats, la juridiction de l’Etat membre d’accueil saisie du contentieux peut les écarter.” Keep the paper trail of a genuine posting: UK contract, UK payslips, HMRC A1, evidence of real activity for the UK employer. It is that file, not the certificate alone, that survives a fraud inquiry.

B. How do I challenge an ICT refusal, an expired-permit dismissal, or a URSSAF bill?

Three disputes recur for posted Britons: the prefecture refuses or will not renew the residence document, the assignment outlasts the permit and the employer reacts, or URSSAF reassesses French contributions. Each has its own forum and clock.

An ICT refusal must state its ground. Compare the letter with the published refusal cases: fraud conviction, three-year ceiling reached, or unauthorised-employment conviction. If the ground is factually wrong, for example the three-year count includes time you spent outside the Union, file a fast administrative appeal to the prefecture itself (recours gracieux) and to its hierarchical superior, then go to the administrative court. Article R. 421-1 of the Administrative Justice Code sets the baseline (CJA Art. R. 421-1): “La juridiction ne peut être saisie que par voie de recours formé contre une décision, et ce, dans les deux mois à partir de la notification ou de la publication de la décision attaquée.” Two months from notification, not from when you feel ready. Attach everything the prefecture said was missing: six-month group seniority proof, management or expert job description, assignment letter with dates, and the host entity’s registration. In Paris and the inner suburbs, expect strict documentary review and bring French translations; the requirement to produce French-language documents runs through the whole posting regime.

If your permit expires mid-assignment, know the protection that saved one foreign worker’s case. On 29 November 2023 the Social Chamber quashed a dismissal validation where the employer had treated an expired card as an immediate sackable offence (Cass. soc., 29 November 2023, No. 22-10.004): “Selon le premier de ces textes, entre la date d’expiration de la carte de résident et la décision prise par l’autorité administrative sur la demande tendant à son renouvellement, dans la limite de trois mois à compter de cette date d’expiration, l’étranger peut justifier de la régularité de son séjour par la présentation de la carte arrivée à expiration. Pendant cette période, il conserve l’intégralité de ses droits sociaux ainsi que son droit d’exercer une activité professionnelle.” File the renewal inside the last two months before expiry, keep the receipt, and show the expired card plus the receipt: your stay remains regular and your right to work continues for up to three months while the prefecture decides. An employer that dismisses you during that window for “no valid permit” dismisses at its own risk before the employment tribunal, the conseil de prud’hommes.

A URSSAF reassessment follows a different track. After an inspection, URSSAF sends a letter of observations, then a formal demand (mise en demeure). Challenge it first in writing with the evidence of genuine posting: surviving UK contract, subordination, A1, SIPSI declaration, representative designation. If URSSAF maintains the bill, the dispute goes to the social-security chamber of the judicial court, and the 20 March 2025 decision above shows how courts reason: treaty first, then the reality of the employment link. Where URSSAF also claims damages in a criminal file, the 9 June 2026 ruling (Cass. crim., 9 June 2026, No. 24-85.090) caps its ambition: relying on a published May 2025 criminal ruling, it holds that URSSAF must prove the investigations caused it extra operating costs beyond its normal duty of checking declarations and contributions. Routine audit costs are its normal mission, not recoverable damages from you.

One forward look for secondees whose temporary posting becomes a life choice. If the assignment extends, your partner joins you, or you buy the Paris flat you first rented, the file changes nature from posting to settlement. At that point the question is no longer which country insures you but which law governs your house if you die owning it. Our guide to choosing English law for a French house under the EU Succession Regulation explains how a British will can elect English law and where French forced heirship still bites. Open that file before the move becomes permanent, not after.

Conclusion

A lawful British posting to France rests on four papers: a genuine surviving UK employment contract, a SIPSI declaration filed before day one with a named French representative, an HMRC A1 certificate covering the assignment up to the 24-month cooperation ceiling, and the right residence document, from short-stay arrangements to the three-year ICT card for intra-group managers and experts. The employer’s French-law duties, equality of treatment, French-language documents and the client’s A1 check, protect you as much as they constrain the company. When a prefecture refuses, a permit expires or URSSAF bills, the remedies exist and the clocks are short: two months for the administrative court, renewal inside the final two months, and a documented fraud-resistant posting file from the first day. Assemble those four papers before the Eurostar, and the assignment stays a career step instead of becoming a case file.