Important ECJ ruling on “substantial activity” under EU Regulations 883/2004 and 987/2009: worldwide activities now count

On 11 December, the European Court of Justice issued a significant judgment in Case C‑743/23 (Moguntia / GKV-Spitzenverband) that changes how applicable social security legislation is determined.

What changed?

• When assessing whether a person pursues a “substantial part” of their activity (≥ 25%) in their home Member State, all work performed worldwide must now be considered – not just activities within the EEA or Switzerland. 
• The calculation method (based on working time and/or remuneration) remains the same, but the scope now includes activities outside the EU/EEA. 

Why does this matter?

Under EU Regulations 883/2004 and 987/2009, a person working in two or more Member States is subject to: 

• The legislation of their state of residence, if they perform a substantial part (≥ 25%) of their activity there; 
• Otherwise, usually the legislation of the state where the employer is established; 
• If the employer is outside the EU, the legislation of the state of residence applies, even without substantial activity there. 

Before the ECJ ruling, the 25% threshold in the state of residence was more often met when employees also worked a substantial part outside the EEA, as those activities were not included in the calculation. 

Including worldwide activities may shift the applicable social security regime from one country to another, with significant implications for compliance.