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.<\/p>\n
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.<\/p>\n
Start with the vocabulary, because the French system uses one word, d\u00e9tachement, for three different situations. A d\u00e9tachement, 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<\/a> states the rule in these terms: “Un employeur \u00e9tabli hors de France peut d\u00e9tacher temporairement des salari\u00e9s sur le territoire national, \u00e0 condition qu’il existe un contrat de travail entre cet employeur et le salari\u00e9 et que leur relation de travail subsiste pendant la p\u00e9riode de d\u00e9tachement.” 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.<\/p>\n 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.<\/p>\n 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.<\/p>\n For assignments of several months inside your own group, the dedicated route is the multi-year residence card marked “salari\u00e9 d\u00e9tach\u00e9 ICT”, named after the EU intra-corporate transfer system. Article L. 421-26 of the Code on the Entry and Stay of Foreigners (CESEDA)<\/a> provides: “L’\u00e9tranger r\u00e9sidant hors de l’Union europ\u00e9enne qui vient en France pour effectuer un d\u00e9tachement temporaire intragroupe, pr\u00e9vu au 2\u00b0 de l’article L. 1262-1 du code du travail, afin d’occuper un poste d’encadrement sup\u00e9rieur ou d’apporter une expertise dans un \u00e9tablissement ou une entreprise du groupe qui l’emploie, et qui justifie d’une anciennet\u00e9 professionnelle d’au moins six mois au sein de ce groupe, se voit d\u00e9livrer une carte de s\u00e9jour pluriannuelle portant la mention ” salari\u00e9 d\u00e9tach\u00e9 ICT ” valable pour la dur\u00e9e du d\u00e9tachement 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<\/a> 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<\/a> organises for ICT families.<\/p>\n 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.<\/p>\n 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<\/a>: 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.<\/p>\n The statute behind that page is Article L. 1262-2-1 of the Labour Code<\/a>. Its first paragraph requires the posting employer to send “une d\u00e9claration, pr\u00e9alablement au d\u00e9tachement, \u00e0 l’inspection du travail du lieu o\u00f9 d\u00e9bute la prestation”. Its second paragraph adds a concrete obligation many British companies miss: the employer “d\u00e9signe un repr\u00e9sentant de l’entreprise sur le territoire national, charg\u00e9 d’assurer la liaison avec les agents mentionn\u00e9s \u00e0 l’article L. 8271-1-2 pendant la dur\u00e9e 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 mentionn\u00e9es aux I et II du pr\u00e9sent article ne pr\u00e9sume pas du caract\u00e8re r\u00e9gulier du d\u00e9tachement.” Filing the declaration does not prove the posting is genuine; inspectors look behind the paperwork at the reality of the employment link.<\/p>\n During the assignment, the employer must also keep French-language documents on site. Article L. 1263-7<\/a> requires the posting employer or its representative to show the inspectorate, at the place of work, “des documents traduits en langue fran\u00e7aise permettant de v\u00e9rifier le respect des dispositions du pr\u00e9sent titre”. Payslips, time records and the A1 social security certificate should be available in French on demand.<\/p>\n The substantive rights you carry with you are called the “hard core”, the noyau dur. Article L. 1262-4<\/a> guarantees you equality of treatment with employees of same-sector companies established in France: “L’employeur d\u00e9tachant temporairement un salari\u00e9 sur le territoire national lui garantit l’\u00e9galit\u00e9 de traitement avec les salari\u00e9s employ\u00e9s par les entreprises de la m\u00eame branche d’activit\u00e9 \u00e9tablies 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.<\/p>\n 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<\/a> sets the scale: “Le montant de l’amende est d’au plus 4 000 \u20ac par salari\u00e9 d\u00e9tach\u00e9 et d’au plus 8 000 \u20ac en cas de r\u00e9it\u00e9ration dans un d\u00e9lai de deux ans \u00e0 compter du jour de la notification de la premi\u00e8re amende. Le montant total de l’amende ne peut \u00eatre sup\u00e9rieur \u00e0 500 000 \u20ac.” 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.<\/p>\n The criminal exposure is real and recent. Concealed employment, travail dissimul\u00e9, is defined by Article L. 8221-5<\/a>: “Est r\u00e9put\u00e9 travail dissimul\u00e9 par dissimulation d’emploi salari\u00e9 le fait pour tout employeur : 1\u00b0 Soit de se soustraire intentionnellement \u00e0 l’accomplissement de la formalit\u00e9 pr\u00e9vue \u00e0 l’article L. 1221-10 , relatif \u00e0 la d\u00e9claration pr\u00e9alable \u00e0 l’embauche ; 2\u00b0 Soit de se soustraire intentionnellement \u00e0 la d\u00e9livrance d’un bulletin de paie ou d’un document \u00e9quivalent d\u00e9fini par voie r\u00e9glementaire, ou de mentionner sur le bulletin de paie ou le document \u00e9quivalent un nombre d’heures de travail inf\u00e9rieur \u00e0 celui r\u00e9ellement accompli, si cette mention ne r\u00e9sulte pas d’une convention ou d’un accord collectif d’am\u00e9nagement du temps de travail conclu en application du titre II du livre Ier de la troisi\u00e8me partie ; 3\u00b0 Soit de se soustraire intentionnellement aux d\u00e9clarations relatives aux salaires ou aux cotisations sociales assises sur ceux-ci aupr\u00e8s des organismes de recouvrement des contributions et cotisations sociales ou de l’administration fiscale en vertu des dispositions l\u00e9gales.” A posting with no SIPSI declaration, no French payroll filings and no contributions is the textbook illustration of the third limb.<\/p>\n 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<\/a>). 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<\/a>): “En pronon\u00e7ant une peine d’interdiction d\u00e9finitive de g\u00e9rer toute entreprise ou toute soci\u00e9t\u00e9, alors que les articles 131-27 du code p\u00e9nal et L. 8224-3 du code du travail, applicables au d\u00e9lit de travail dissimul\u00e9, limitent une telle interdiction aux entreprises commerciales ou industrielles et aux soci\u00e9t\u00e9s commerciales, la cour d’appel a m\u00e9connu le texte susvis\u00e9 et le principe ci-dessus rappel\u00e9.” 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.<\/p>\n 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<\/a>): “Il r\u00e9sulte du premier de ces textes qu’est interdit le fait de recourir sciemment, directement ou par personne interpos\u00e9e, aux services de celui qui exerce un travail dissimul\u00e9.” And: “Il se d\u00e9duit du second que la personne morale qui contracte avec une entreprise \u00e9tablie ou domicili\u00e9e dans un autre Etat membre de l’Union europ\u00e9enne doit, dans tous les cas, se faire remettre par celle-ci le certificat A1 attestant de la r\u00e9gularit\u00e9 de la situation sociale du cocontractant au regard du r\u00e8glement (CE) n\u00b0 883\/2004 du Parlement europ\u00e9en et du Conseil du 29 avril 2004 portant coordination des syst\u00e8mes de s\u00e9curit\u00e9 sociale pour chacun des travailleurs d\u00e9tach\u00e9s 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.<\/p>\n The starting point in France is affiliation by territory. Article L. 111-2-2 of the Social Security Code<\/a> provides: “Sous r\u00e9serve des trait\u00e9s et accords internationaux r\u00e9guli\u00e8rement ratifi\u00e9s ou approuv\u00e9s et des r\u00e8glements europ\u00e9ens, sont affili\u00e9es \u00e0 un r\u00e9gime obligatoire de s\u00e9curit\u00e9 sociale dans le cadre du pr\u00e9sent code, quel que soit leur lieu de r\u00e9sidence, toutes les personnes : 1\u00b0 Qui exercent sur le territoire fran\u00e7ais : a) Une activit\u00e9 pour le compte d’un ou de plusieurs employeurs, ayant ou non un \u00e9tablissement 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 r\u00e9serve des trait\u00e9s et accords internationaux”, subject to international treaties and agreements.<\/p>\n 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<\/a>: 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<\/a>: 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.<\/p>\n 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<\/a>): “Selon le premier de ces textes, toute personne exer\u00e7ant une activit\u00e9 salari\u00e9e ou non sur le territoire fran\u00e7ais est obligatoirement affili\u00e9e au r\u00e9gime obligatoire de s\u00e9curit\u00e9 sociale fran\u00e7ais, sous r\u00e9serve des trait\u00e9s et accords internationaux.” The Court then held (Cass. 2nd civ., 20 March 2025, No. 23-12.264<\/a>) that a local mission contract could not prove a posting where the lower court gave “motifs impropres \u00e0 caract\u00e9riser que M. [R] \u00e9tait d\u00e9tach\u00e9 de la soci\u00e9t\u00e9 [4] avec maintien d’un lien contractuel et de subordination avec cette derni\u00e8re”. 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.<\/p>\n 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<\/a>) recalled the Court of Justice’s Bouygues ruling of 14 May 2020 (C-17\/19): “les formulaires de d\u00e9tachement, dits certificats E 101 et A1, s’imposent aux juridictions de l’Etat sur le territoire duquel les travailleurs exercent leurs activit\u00e9s uniquement en mati\u00e8re de s\u00e9curit\u00e9 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.<\/p>\n 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<\/a>) the Criminal Chamber stated: “La mise en oeuvre de la proc\u00e9dure de dialogue et de conciliation pr\u00e9vue au r\u00e8glement (CE) n\u00b0 883\/2004 du Parlement europ\u00e9en et du Conseil du 29 avril 2004 sur la coordination des syst\u00e8mes de s\u00e9curit\u00e9 sociale, qui constitue un pr\u00e9alable obligatoire \u00e0 un \u00e9ventuel constat d\u00e9finitif de fraude par les autorit\u00e9s comp\u00e9tentes de l’Etat d’accueil, lorsque les poursuites pour travail dissimul\u00e9 ont \u00e9t\u00e9 engag\u00e9es pour d\u00e9faut de d\u00e9clarations aux organismes de protection sociale (CJUE, arr\u00eat 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<\/a>): “le fait pour l’institution comp\u00e9tente de l’Etat membre d’\u00e9mission de confirmer le bien-fond\u00e9 de certificats A1 sans se prononcer sur les \u00e9l\u00e9ments concrets soumis par l’institution comp\u00e9tente de l’\u00c9tat membre d’accueil qui donnent \u00e0 penser que ces certificats ont \u00e9t\u00e9 obtenus ou invoqu\u00e9s de mani\u00e8re frauduleuse ne peut \u00eatre assimil\u00e9 \u00e0 un r\u00e9examen effectif. Dans ce cas, et d\u00e8s lors qu’elle constate le caract\u00e8re frauduleux de ces certificats, la juridiction de l’Etat membre d’accueil saisie du contentieux peut les \u00e9carter.” 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.<\/p>\n 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.<\/p>\n 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<\/a>): “La juridiction ne peut \u00eatre saisie que par voie de recours form\u00e9 contre une d\u00e9cision, et ce, dans les deux mois \u00e0 partir de la notification ou de la publication de la d\u00e9cision attaqu\u00e9e.” 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.<\/p>\n 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<\/a>): “Selon le premier de ces textes, entre la date d’expiration de la carte de r\u00e9sident et la d\u00e9cision prise par l’autorit\u00e9 administrative sur la demande tendant \u00e0 son renouvellement, dans la limite de trois mois \u00e0 compter de cette date d’expiration, l’\u00e9tranger peut justifier de la r\u00e9gularit\u00e9 de son s\u00e9jour par la pr\u00e9sentation de la carte arriv\u00e9e \u00e0 expiration. Pendant cette p\u00e9riode, il conserve l’int\u00e9gralit\u00e9 de ses droits sociaux ainsi que son droit d’exercer une activit\u00e9 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.<\/p>\n 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<\/a>) 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.<\/p>\nB. What must my UK employer declare on SIPSI, and what happens if it files nothing?<\/h3>\n
II. Which country’s social security covers me while posted, and who pays if things go wrong?<\/h2>\n
A. Do I stay on UK National Insurance or join the French system while posted?<\/h3>\n
B. How do I challenge an ICT refusal, an expired-permit dismissal, or a URSSAF bill?<\/h3>\n