On 4 September 2025, the Court of Justice of the European Union issued a judgment clarifying how to determine whether an employee performs a “substantial part” of their work in their Member State of residence under Regulation (EC) No 883/2004 and its implementing regulation No 987/2009.[1]

According to Article 13(1) of Regulation 883/2004, an employee working in multiple Member States is subject to the social security legislation of their country of residence if they perform a substantial part of their activity there, or if they work for multiple employers in different Member States. Otherwise, the applicable legislation is that of the employer’s registered office.

The judgment confirms that “substantial part” must be assessed solely based on working time and/or remuneration, as defined in Article 14(8) of Regulation 987/2009. If less than 25% of either criterion is met in the residence state, the activity is not considered substantial there.

Other factors – such as the nature of the work or location of clients – are irrelevant, even if some language versions of the regulation suggest otherwise. The assessment must be based on the projected situation over the next 12 months from the start of cross-border work. It is the responsibility of the competent social security institution to determine whether the 25% threshold is met, based on an overall evaluation of time and pay.

[1] European Court of Justice 04 September 2025, ECLI:EU:C:2025:662.