Short answer
EU coordination normally makes only one Member State's social-security legislation applicable at a time. The default for an employee is the State where the work is actually performed. A genuine temporary posting can preserve the sending State's system for up to 24 months. A person who habitually works in two or more Member States follows a different rule: the residence State applies if a substantial part of the activity is performed there; otherwise the employer's registered-office rules usually decide.
Remote work changes the physical work location and can therefore change the answer. The contract's governing law, payroll location, employer preference and nationality do not by themselves select social-security legislation. Competent authorities can agree an Article 16 exception in a person's interest, but the parties cannot create one privately.
An A1 certificate records the applicable legislation and generally binds other Member States until the issuing institution withdraws or invalidates it. A State that disagrees must seek review and use the institutional dialogue process. Meanwhile, provisional coverage prevents a gap; contributions and benefits are reconciled retroactively once the conflict is resolved.
One person, one applicable system
Regulation (EC) No 883/2004 coordinates national systems rather than replacing them with an EU scheme. Its starting rule is categorical:
“Persons to whom this Regulation applies shall be subject to the legislation of a single Member State only.” —
Once the conflict rules identify that State, its national law determines contribution rates, insured status and benefits, subject to EU equality, aggregation and export rules.
For an ordinary employee working in one State, Article 11(3)(a) selects the State where the employed activity is pursued. The employer may be established elsewhere. Special categories—including civil servants, flight crew, seafarers and people receiving certain unemployment benefits—have tailored rules.
A temporary posting is not ordinary multi-state work
Article 12 allows an employee sent temporarily by an employer that normally carries on activities in the sending State to remain under that State's legislation. The anticipated work abroad must not exceed 24 months, and the person must not replace another posted person.
There must be a real direct employment relationship during the posting and substantial employer activity in the sending State, not merely internal administration. A succession of workers cannot use separate certificates to fill the same continuing foreign role. Material changes should be reported.
Posting is distinguished from a person who normally works in several States. A one-off temporary assignment differs from an enduring two-country schedule.
Remote work can create multi-state activity
An employee who regularly works at home in one Member State and at the employer's premises in another normally pursues activities in two States. Article 13(1) first asks whether the employee performs a substantial part in the State of residence.
Regulation (EC) No 987/2009 explains that “substantial” need not be the majority. Working time and remuneration are indicators; a share below 25% indicates that a substantial part is not performed there. Institutions consider the expected pattern over the following 12 calendar months, not one unusual week.
If a substantial part is performed in the residence State, that State's legislation applies. If not, the outcome depends on the employer structure:
- one employer: normally the State of its registered office or place of business;
- multiple employers all established in one State: that State;
- employers in the residence State and one other State: generally the other State; or
- at least two employers in different States outside the residence State: the residence State.
“Residence” is determined from the person's habitual centre of interests using the EU coordination criteria, not simply a registered address.
For self-employed multi-state activity, the residence State applies if a substantial part occurs there; otherwise the centre of interest of the activities controls. Mixed employed and self-employed work generally follows the State selected for the employed activity.
Article 16 can support an agreed exception
Two or more Member States' competent authorities may agree an exception to Articles 11–15 in the interest of a person or category. This is the legal mechanism often used for agreed cross-border telework frameworks.
It is not an employee or employer election. The relevant authorities must both participate, and States not party to a particular framework are not bound by it. The Administrative Commission's Decision H14 published EU guidance on telework applicable from 1 July 2023, but the applicable Article 16 arrangement and participating States still have to be checked for the worker concerned. Before relying on a telework exception, check participation, residence, employer establishment, working percentage, request timing and the issued A1.
What an A1 certificate does
An A1 certificate is issued by the competent institution to state which legislation applies. Article 5 of Regulation 987/2009 gives cross-border documents strong effect:
They “shall be accepted by the institutions of the other Member States for as long as they have not been withdrawn or declared to be invalid by the Member State in which they were issued.” —
Case C-620/15, A-Rosa Flussschiff, confirmed the binding logic for the predecessor E 101 certificate. The host institution or court cannot normally declare the certificate invalid simply because it assesses the facts differently. Mutual trust requires the issuing institution to reconsider its grounds when another institution supplies doubts and evidence.
Joined Cases C-410/21 and C-661/21 confirmed that an A1 remains binding while the issuing institution has only provisionally suspended its effects and has not made a final withdrawal decision. The Court also held that a Community road-transport licence does not conclusively establish the employer's registered office for Article 13: the institution must identify where the undertaking's essential decisions are taken and its central administration operates.
The certificate is not a private guarantee that false facts become true. Employers should preserve work schedules, posting instructions, establishment evidence and residence information, and notify material changes.
How conflicting registrations are corrected
A person normally working in two or more Member States must inform the institution designated by the residence State. That institution makes an initial determination and informs the institutions of the other work States. Unless challenged within two months, the determination becomes definitive; if institutions disagree, they seek agreement using the dialogue and conciliation process.
Article 6 protects the person during disagreement. One State's legislation applies provisionally according to stated priorities, and provisional benefits remain available. When the competent State is finally established, it is treated as competent retroactively. Institutions then transfer or refund provisional contributions and settle benefit payments. This mechanism is designed to avoid both double insurance and a coverage vacuum.
The practical response to duplicate registrations is coordinated correction, not simply stopping payments in one State. The worker and employer should provide the same timeline to both institutions and request an applicable-legislation decision and A1 review.
Fraud is a narrow exception, not a shortcut
Case C-359/16, Altun, recognised that a host court may disregard a fraudulently obtained certificate in limited circumstances. The fraud has an objective element—the Article 12 conditions were not met—and a subjective element: intentional evasion through false facts or concealment. The host institution must first present concrete evidence to the issuing institution and give it a reasonable opportunity to review.
Joined Cases C-370/17 and C-37/18, Vueling Airlines, emphasised that dialogue is mandatory before a definitive host-State fraud finding. Even concrete evidence does not permit unilateral disregard without promptly initiating the procedure and assessing the issuing institution's response. The accused must also receive fair-trial safeguards.
Case C-421/23, decided in 2025, applies the same cooperation discipline where documents presented as A1 certificates were not authentic. The host authority still had to initiate the Article 76(6) dialogue, but the host court was not bound after the purported issuing institution confirmed that it had never issued the documents.
That distinction matters for ordinary errors. A changed work pattern, conflicting institutional interpretation or incorrect registration should be corrected through review, withdrawal and retroactive settlement. It is not automatically fraud.
A cross-border coverage checklist
- Map every physical work location and expected percentage over 12 months.
- Identify genuine residence under the coordination criteria.
- Separate temporary posting from habitual alternating work.
- Verify the employer's real activities and registered office in the relevant State.
- Check special rules for flight crew, seafarers, civil servants and mixed work.
- Determine whether an Article 16 telework or individual exception actually applies.
- Notify the residence institution before or when multi-state work begins.
- Obtain and monitor the A1; report changes that affect its factual basis.
- If institutions disagree, request formal review and provisional coverage rather than paying selectively.
- Reconcile contributions, benefits and payroll after the final retroactive determination.
Conclusion
EU law selects one social-security system from the worker's actual pattern. The work State is the default; genuine posting, habitual multi-state activity and agreed Article 16 exceptions alter it under specific conditions. Regular home working can therefore move coverage even when the employment contract and employer stay unchanged.
An A1 certificate makes the determination portable and binding, but it is corrected by the issuing institution through structured cooperation. Conflicting registrations should lead to provisional protection and retroactive institutional settlement. Unilateral disregard is reserved for a narrowly evidenced fraud process, not ordinary disagreement or administrative error.
Sources
This article provides general legal information, not advice for a specific matter. Laws, facts and procedural deadlines may change the result. Follow the linked primary sources and obtain local advice before acting.