The case concerns the so-called "Portable Documents A1", which are issued to workers temporarily posted to another Member State and to people working in several countries at the same time. The document certifies that these workers pay their social security contributions in the issuing Member State.
According to EU law, consistently confirmed by the Court of Justice, other Member States are obliged to accept such documents as long as they have not been withdrawn or declared invalid by the issuing Member State.
In order to guard against abuse or fraud, there are specific and clearly established EU rules to verify that these documents are valid. These rules fix deadlines for other Member States to respond to requests of verification and provide a conciliation procedure in cases of disagreement.
However, Belgian law enables local enforcement authorities in Belgium to decide unilaterally not to recognise certificates from other Member States, without making use of these procedures.
The Commission sent a ´reasoned opinion´ to Belgium concerning this matter in September 2014.