A registered letter arrives at your house in the Lot, or a commissaire de justice — the enforcement officer formerly called the huissier — knocks with an envelope. Inside sits a contrainte: a formal enforcement order for several thousand euros of unpaid cotisation subsidiaire maladie (CSM, the yearly subsidiary health contribution levied on inactive residents covered by the French health system). You never paid the underlying bill, or never even saw it, and now the document speaks of opposition periods, competent courts, and enforcement. For a British resident after Brexit, unfamiliar with French recovery vocabulary and unsure which deadline bites first, this is the moment files are lost — not because the debt was unanswerable, but because the wrong remedy was filed, or the right one a day late.

This guide walks you through the enforcement chain in the order it actually strikes. It starts upstream, with the CSM call (appel de cotisation) and the one-month formal warning (mise en demeure) that must precede any enforcement; it dissects the contrainte itself, whose missing compulsory mentions can annul it; and it sets out the two parallel defences — the fifteen-day court opposition (opposition à contrainte) that goes straight to the judge, and the challenge to the CSM bill underneath, through the fund’s appeals board and back to court. Every decisive rule below is quoted from a statute or a judgment verified for this article, with its official link in the same sentence. The companion piece on the CSM bill itself — who owes it, how Urssaf computes it, and how to contest the assessment — is examined in our earlier guide (see British Early Retiree in France: Private Insurance, PUMA Entry and the CSM Bill — and How to Challenge a Refusal or a Levy); this article takes over where that one stops, when the bill has turned into enforcement.

I. Reading the enforcement chain before the bailiff knocks

A. From the CSM call to the mise en demeure: the one-month warning you must not ignore

Enforcement never starts with the contrainte. It starts a year earlier, with the CSM itself. Article L380-2 of the Code de la sécurité sociale provides that “Les personnes mentionnées à l’article L. 160-1 sont redevables d’une cotisation annuelle lorsqu’elles remplissent les conditions suivantes”, namely low French professional income for the year and no pension, annuity or unemployment benefit for either spouse. The levy hits the classic British early-retired household living on dividends, rental income and capital gains, and Article L380-2 of the Code de la sécurité sociale adds the timing rule that shapes every enforcement file: “La cotisation est recouvrée l’année qui suit l’année considérée”. Your 2025 position is therefore billed in 2026, on figures the tax office transmits automatically to the recovery body — and a move back to Britain, a late tax return, or post redirected to a letting agent never suspends that billing. If the call (appel de cotisation) went to an old address while you were between homes, the debt still grew, and the enforcement you hold today is its end product. Check the underlying call first, using the computation rule in Article D380-1 of the Code de la sécurité sociale, which sets the degressive formula: “Montant de la cotisation = 6,5 % × (A-0,5 × PASS) × [1-R/ (0,2 × PASS)]”, where A is the capital-income base capped at eight times the yearly ceiling (plafond annuel de la sécurité sociale, PASS) and R the household’s French professional income. Recompute it from your final tax notice (avis d’impôt) before anything else: a surprising number of enforcement files collapse at this step, because the base year, the PASS value, the joint-income split, or the pension exclusion was misapplied.

Between the unpaid call and the contrainte stands a mandatory warning. Article R133-3 of the Code de la sécurité sociale provides that “Si la mise en demeure ou l’avertissement reste sans effet au terme du délai d’un mois à compter de sa notification, les directeurs des organismes créanciers peuvent décerner” a contrainte. The one-month period runs from the notification of the mise en demeure, and no enforcement order may lawfully issue before it expires. In practice this gives you two verifications the moment any enforcement lands. First, was there a prior mise en demeure, properly notified, and did a full month pass without effect? If the recovery body jumped straight from the call to the contrainte, or the warning letter cannot be produced with its date of receipt, the chain is defective. Second, did you pay, query, or contest within that month? A payment under protest, a written request for the detailed computation, or a complaint to the friendly-appeals board (commission de recours amiable, CRA) inside that window often freezes escalation and creates the paper trail the judge will later read. Keep the warning letter, its envelope, and every proof of your response: enforcement litigation is won on dates and receipts, never on recollection.

B. The contrainte itself: service, compulsory mentions, and the nullity weapon

The contrainte is not a reminder. It is an enforceable order that, unchallenged, authorises seizure. That is why the statute polices its form strictly. Article R133-3 of the Code de la sécurité sociale states that “La contrainte est signifiée au débiteur par acte de commissaire de justice ou par lettre recommandée avec demande d’avis de réception.” Either personal service by the enforcement officer or registered letter with acknowledgement will do — but an ordinary letter, an email, or a message in a portal inbox does not. If your contrainte arrived by any of those informal routes, say so in your challenge and demand proof of the date of receipt, because every subsequent deadline is counted from notification or service, and the body must be able to prove that date.

Form continues to matter on the face of the document. Article R133-3 of the Code de la sécurité sociale provides that “A peine de nullité, l’acte de commissaire de justice ou la notification mentionne la référence de la contrainte et son montant, le délai dans lequel l’opposition doit être formée, l’adresse du tribunal compétent et les formes requises pour sa saisine.” Four mentions under penalty of nullity: the order’s reference and amount, the opposition period, the court’s address, and the required form of application. Read your document against that list with a ruler. A contrainte that omits the court, misstates the period, or hides the forms of referral is voidable, and nullity of the order kills the enforcement even if the underlying CSM was perfectly due. British recipients should pay particular attention to the address given: for debtors living abroad, the competent court sits where the creditor body sits, and an address pointing you to the wrong town is not a detail. Plead nullity expressly in your opposition — judges do not always raise formal defects on their own — and attach a clean copy of the order exactly as received, envelope and acknowledgement included.

Two final features of the order shape your strategy. The officer must warn the creditor body quickly: the same article notes that the commissaire de justice informs the body of the date of service within eight days, which fixes the evidential starting point both sides will use. And the endgame is defined in advance: under Article R133-3 of the Code de la sécurité sociale, “La décision du tribunal, statuant sur opposition, se substitue à la contrainte. Elle est exécutoire de droit à titre provisoire.” The court’s judgment replaces the order — which is why winning on the merits ends enforcement outright — but it is provisionally enforceable by right, meaning an appeal does not automatically suspend payment. Factor that into negotiation: even a strong file may warrant payment under protest to stop surcharges while the case runs.

II. Stopping enforcement and killing the underlying bill

A. The fifteen-day opposition: form, forum, and what the judge decides

The opposition to a contrainte is the emergency exit, and it closes fast. Article R133-3 of the Code de la sécurité sociale provides that “Le débiteur peut former opposition par inscription au secrétariat du tribunal compétent dans le ressort duquel il est domicilié ou pour les débiteurs domiciliés à l’étranger, au secrétariat du tribunal compétent dans le ressort de l’organisme créancier par lettre recommandée avec demande d’avis de réception adressée au secrétariat dudit tribunal dans les quinze jours à compter de la notification ou de la signification.” Fifteen days from notification or service — not a month, not two months — and the clock starts on the date the creditor can prove, which returns you to the service check in Part I. A debtor living in France registers the opposition at the court registry (secrétariat, in practice the greffe) of the court for their home; a debtor who has returned to Britain sends it by registered letter with acknowledgement to the registry of the court where the creditor body sits. Do both the counting and the routing on the day the order arrives: a day-sixteen filing is inadmissible however strong the merits, and a filing at the wrong registry wastes the days you cannot spare.

Form is as strict as timing. Article R133-3 of the Code de la sécurité sociale continues: “L’opposition doit être motivée ; une copie de la contrainte contestée doit lui être jointe.” A reasoned opposition with a copy of the contested order attached. “Motivée” means stating grounds, not feelings: identify the order (reference, date, amount), list each plea in numbered paragraphs — defective service, missing compulsory mentions, no prior mise en demeure or an unexpired one-month period, wrong amount, wrong person, wrong year, pension exclusion unexamined — and attach the exhibits that prove each one. The court registry then warns the creditor body within eight days of receiving the opposition, so the machinery engages quickly on both sides. Unlike a challenge to the CSM assessment itself, this route goes directly to the judge: no prior complaint to the appeals board is required before opposing a contrainte, because the statute sends you straight to the registry. Do not therefore burn your fifteen days writing to Urssaf’s complaints department instead of the court; you may write to both, but the court filing is the one that stops the clock.

Before the judge — the social chamber (pôle social) of the judicial court (tribunal judiciaire) — the opposition opens the whole file. The tribunal examines the regularity of the enforcement (service, mentions, prior warning, time limits) and the merits of the debt (liability, base, rate), then substitutes its own judgment for the order. Total victory annuls enforcement and, where the debt itself falls, ends the matter; partial victory corrects the amount and enforcement continues for the remainder. Remember the provisional sting noted above: the judgment is enforceable as of right on a provisional basis, so ask expressly, where the rules allow, for a stay or for restitution of sums paid under protest, and keep paying disputed surcharges under protest while an appeal runs rather than letting them compound. In Paris and the inner suburbs, where Urssaf Île-de-France dockets move slowly but bailiff service is swift, file at the registry in person where possible and keep the stamped copy: it is your only certain proof of date, and everything in this procedure turns on dates.

B. Challenging the CSM underneath: exemptions, base errors, and the judge’s control

Killing the order is half the battle; killing the debt is the other half, and a parallel challenge to the CSM assessment itself is usually indispensable — otherwise a corrected contrainte simply returns next quarter. The assessment challenge follows the ordinary social-security ladder, starting with the friendly-appeals board. Article L142-4 of the Code de la sécurité sociale makes contentious appeals in these matters subject to a prior administrative complaint, and Article R142-1 of the Code de la sécurité sociale sends it to the board with a strict clock: “Cette commission doit être saisie dans le délai de deux mois à compter de la notification de la décision contre laquelle les intéressés entendent former une réclamation.” Two months from the notification of the CSM call — a longer fuse than the fifteen-day opposition, but one that often already expired while the file slept in a drawer. If it has, say so honestly and attack the enforcement on its own defects instead; if it has not, file the board complaint by registered letter immediately, even while the court opposition runs, because the two routes are independent and each protects a different flank.

On substance, the statute hands you three ready-made defences, and the courts apply them as written. First, the status exclusions. Article L380-2 of the Code de la sécurité sociale exempts anyone who, in the year considered, earned French professional income at or above the decree threshold or received any retirement or invalidity pension, annuity or unemployment benefit — each condition examined for both spouses. A small British personal-pension annuity starting mid-year, a French survivor’s pension (pension de réversion), or a few months of consultancy profit can each take the whole year out of charge. Second, the base. The same article seats the levy on land income, investment income, capital gains and non-professional business profits measured under tax-code rules, with a lifestyle backstop where declared income understates means — and Article D380-5 of the Code de la sécurité sociale defaults undivided joint income to a fifty-fifty split between spouses unless the debtor proves the exact shares. Recompute the 6.5 per cent formula from the final tax notice, allocate joint dividends with bank and broker statements, and challenge every classification error: the wrong base year, the wrong PASS, a misapplied cap, or a missing prorata for part-year cover each go to the legality of the call. Third, residence itself: the levy only attaches to persons covered by residence-based health protection for the year, so a year spent mostly outside France with no stable French home cannot generate it — proved, as always, with dated presence evidence, since Article D160-2 of the Code de la sécurité sociale demands proof that newcomers “doivent produire un justificatif démontrant qu’elles résident en France de manière ininterrompue depuis plus de trois mois”, and the Cour de cassation applies that residence test without distinction of nationality (see Cass. 2e civ., 3 June 2021, No. 20-10.687, published in the Bulletin, holding that “Ces dispositions, qui s’appliquent sans distinction de nationalité à toute personne qui, n’exerçant pas d’activité professionnelle, peut bénéficier, en cas de maladie ou de maternité, de la prise en charge de ses frais de santé par l’assurance maladie, revêtent un caractère limité et répondent aux exigences de la gestion d’un système d’assurance maladie étendu à l’ensemble de la population active et résidente”).

The leading judgment on the levy itself shows judges exactly where to look — and where challengers waste their time. In Cass. 2e civ., 28 January 2021, No. 19-25.853, a first-instance judge had annulled a 2016 CSM call on the ground that the 2017 implementing decrees were unknowable when the year ran; the Cour de cassation quashed that reasoning and restated the architecture: “Selon le deuxième de ces textes, les personnes mentionnées à l’article L. 160-1 sont redevables d’une cotisation annuelle dont les conditions d’assujettissement, les modalités de détermination de l’assiette et le taux sont fixés par les trois derniers.” Arguments that the levy is too novel or obscure to be lawful therefore fail — but the Court’s own method points to the pleas that succeed: liability conditions, base determination, and rate as fixed by the regulatory articles. Plead the wrong base, the wrong rate, the mis-split joint income, or the unexamined pension, with documents, and you speak the Court’s language. If the board rejects the complaint, the dispute goes to the social chamber of the judicial court and then the court of appeal, under the ordinary two-month discipline that Article R421-1 of the Code de justice administrative states as follows: “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.” Run both tracks together — fifteen-day opposition against the order, two-month board complaint against the assessment — pay under protest where enforcement threatens, claim back surcharges expressly if the debt falls, and diary every spouse’s change of status, since pensions and earnings move the following year’s analysis for the whole household.

Make the file court-ready from day one by assembling one binder in this order: the contested contrainte with its envelope and acknowledgement of receipt; the earlier mise en demeure with its own proof of receipt, so the one-month interval can be measured; the underlying CSM call with your recomputation of the 6.5 per cent formula beside it; the tax notice for the year considered with the passages feeding the base highlighted; proof of any pension, annuity, unemployment benefit or French professional income for that year, for both spouses, since each of these can take the year out of charge; presence evidence where residence itself is disputed; and copies of everything already sent to the recovery body or the appeals board, with dates. Number every exhibit, reference each number in the opposition or complaint, and never send an original. This discipline does more than impress: where the clock is counted in fifteen days, a complete file lets the judge decide on the papers without adjournment, and where the board has two months to answer, it removes every pretext for asking you twice for the same document.

Conclusion

A contrainte for unpaid CSM is frightening but legible: a call, a one-month warning, an order with four compulsory mentions, a fifteen-day dash to the court registry, and behind it all a levy whose exemptions and arithmetic can be checked line by line. The British resident who reads the order against the statute, files the opposition in time and in the right forum, and challenges the assessment in parallel holds every card the procedure offers. Act on dates, keep every envelope, and make the recovery body prove each link of its own chain. Opposition within fifteen days, complaint within two months, nullity pleaded expressly, and the base recomputed to the euro: that is the complete defence, and it fits on a single page pinned above your desk.