The admissibility of an action for failure to fulfil obligations against a Member State cannot be called in question by the fact that the Court may find it necessary, in the context of that action, to classify, with regard to EU law, another Member State’s body of rules. Nor can such clarification give rise to an infringement of the procedural rights of the latter Member State, an intervener in the proceedings.

It follows from the principle of sincere cooperation, laid down in Article 4(3) TEU, that every Member State, for the purposes of the declarations covered by Article 5 of Regulation No 1408/71 on the application of social security schemes to employed persons, to self-employed persons and to members of their families moving within the Community and Article 9(1) of Regulation No 883/2004 on the coordination of social security systems, must carry out a proper assessment of its own social security regimes and, if necessary, following that assessment, declare them as falling within the scope of those regulations. It also follows from this principle that the other Member States are entitled to expect that the Member State concerned had fulfilled those obligations.

Thus, those declarations create a presumption that the national laws in a declaration under Article 5 of Regulation No 1408/71 and Article 9 of Regulation No 883/2004 fall within the material scope of those regulations and bind, in principle, the other Member States. Conversely, where a Member State has refrained from declaring a national law under those regulations, the other Member States can, generally, infer from it that that law does not fall within the material scope of those regulations.

In addition, as long as the declarations made by a Member State are not amended or withdrawn, the other Member States must take them into account. It is incumbent upon the Member State which made the declaration to reconsider its merits and, if necessary, to amend it if another Member State expresses doubts as to the correctness of those declarations.

This finding does not, however, mean that a Member State is denied any chance of responding when it is aware of information that raises doubts regarding the declarations made by another Member State.

In the first place, if the declaration raises questions and if the Member States cannot reach agreement, in particular regarding the classification of laws or schemes within the scope of Regulations Nos 1408/71 and 883/2004, they may turn to the Administrative Commission, mentioned in Articles 80 and 81 of Regulation No 1408/71 and in Articles 71 and 72 of Regulation No 883/2004. In the second place, if that commission does not succeed in reconciling the points of view of the Member States on the question of the legislation applicable in the case in point, it is, where appropriate, for the Member State doubting the correctness of a declaration by another Member State to tell the Commission or, as a last resort, bring proceedings under Article 259 TFEU in order for the Court to examine, in the context of those proceedings, the question of the applicable legislation.

On the other hand, it does not follow from either Article 5 of Regulation No 1408/71 or Article 9 of Regulation No 883/2004 that it is the duty of the Member States, other than that which introduced that law or regulation but did not declare it, to determine on their own initiative whether that law or regulation must be regarded as falling within the material scope of the regulations concerned.

(see paras 37-41, 44)