On 23 January 2025, the European Court of Justice (ECJ) issued a landmark judgment in Case C-421/23, clarifying how EU social security rules apply to posted workers when A1 certificates are fraudulent or invalid.
As national authorities now begin to implement the ruling, the case is shaping cross-border enforcement practices and compliance obligations for employers across the EU.
An A1 Certificate is issued by a national social security authority and confirms which country’s social security legislation applies to a worker. It is relevant in cases involving:
The certificate helps:
Without an A1, employers risk high fines, delays, or double liability in different jurisdictions.
The case arose from Belgium, where a Portuguese contractor employed 650 Portuguese workers on Belgian construction sites (2012–2017). The workers presented A1 certificates, but Belgium discovered they had not been issued by the Portuguese social security authority.
Belgian courts convicted the employer for fraud relating to social security contributions and false documents. On appeal, the case was referred to the ECJ for interpretation of EU law.
The ECJ made three critical rulings:
The judgment strengthens the legal value of A1 certificates, while clarifying how fraud should be addressed:
For employers, this means:
The 2025 ECJ ruling confirms that A1 certificates continue to carry strong legal protection, but also underscores that fraudulent use will trigger enforcement and penalties. Employers posting workers within the EU must:
As national authorities now begin to implement the ruling, the case is shaping cross-border enforcement practices and compliance obligations for employers across the EU.
What Is an A1 Certificate and Why Does It Matter?
An A1 Certificate is issued by a national social security authority and confirms which country’s social security legislation applies to a worker. It is relevant in cases involving:
- Posted workers sent temporarily to another EU country.
- Business travellers on short-term assignments.
- Self-employed workers or employees performing duties in multiple EU countries simultaneously.
The certificate helps:
- Ensure legal certainty on which country’s system applies.
- Avoid double contributions.
- Support compliance for immigration and posted worker notifications.
Without an A1, employers risk high fines, delays, or double liability in different jurisdictions.
Background of the 2025 Case
The case arose from Belgium, where a Portuguese contractor employed 650 Portuguese workers on Belgian construction sites (2012–2017). The workers presented A1 certificates, but Belgium discovered they had not been issued by the Portuguese social security authority.
Belgian courts convicted the employer for fraud relating to social security contributions and false documents. On appeal, the case was referred to the ECJ for interpretation of EU law.
ECJ’s Key Findings in Case C-421/23
The ECJ made three critical rulings:
- Applicability of Regulation 883/2004
- EU social security coordination rules apply even when A1 certificates are fraudulent.
- Workers must remain covered by only one country’s system at a time, to avoid uncertainty.
- National courts must check whether the legal posting conditions were satisfied.
- Obligation for Dialogue Between Authorities
- If fraud is suspected, member states must use the structured dialogue and conciliation procedure set out in Regulation 883/2004.
- This reinforces the principle of sincere cooperation between states.
- Invalid Certificates
- When the issuing authority confirms it never issued the certificate (or that it is invalid), other states’ courts are no longer bound by it.
- No further consultation is required in such cases.
What This Means for Employers
The judgment strengthens the legal value of A1 certificates, while clarifying how fraud should be addressed:
- Host country authorities cannot simply ignore an A1, even if fraud is suspected.
- Instead, they must trigger the conciliation process with the issuing state.
- Only when the home authority confirms invalidity can host courts set aside the certificate.
For employers, this means:
- A1 certificates remain a crucial safeguard against unexpected social security claims.
- Ensuring valid issuance is more important than ever, given rising enforcement.
- Inspections are increasing across Europe, with national authorities making use of the Electronic Exchange of Social Security Information (EESSI), a digital platform enabling real-time cross-border checks.
Key Takeaway
The 2025 ECJ ruling confirms that A1 certificates continue to carry strong legal protection, but also underscores that fraudulent use will trigger enforcement and penalties. Employers posting workers within the EU must:
- Secure valid A1 certificates before assignments begin.
- Keep detailed records to evidence compliance.
- Prepare for heightened inspections as EU authorities rely more heavily on digital verification systems.
. But seriously, the new ruling makes it harder for shady contractors to play the “we didn’t know” card. Good for legit businesses that actually follow EU posting rules.
I didn’t realise they actually had a digital system now (EESSI??). That’s sooo cool and high-tech!! At least it’ll make things more transparent across borders!! 
