On 11 July 2018, in the case of the European Commission -v – Kingdom of Belgium (C-356/15) the ECJ ruled that national laws that make it possible to undermine an A1 certificate by unilaterally subjecting a worker to its social security legislation, are not compatible with the principle of sincere cooperation between Member States.
Therefore, the Belgian courts may only unilaterally set aside an A1 certificate within the narrow limits set out in the Altun case (see Update from April 2018).