Even in the case of a manifest error of assessment of the conditions governing the application of Regulation No 883/2004 on the coordination of social security systems, as amended by Regulation No 465/2012, and even if it were established that the conditions under which the posted workers carry out their activities clearly do not fall within the material scope of the provision on the basis of which the A 1 certificate was issued, the procedure to be followed in order to resolve any dispute between the institutions of the Member States concerned as regards the validity or the accuracy of an A 1 certificate must be complied with.

If it were accepted that the Member State in which a worker is posted could adopt legislation that allowed its own institutions to have an A 1 certificate declared invalid unilaterally by bringing proceedings before a court of that Member State, there would be a risk that the system based on the duty of sincere cooperation between the competent institutions of the Member States would be undermined. Therefore, as long as it has not been withdrawn or declared invalid, an A 1 certificate is to be accepted, as a rule, in the internal legal order of the Member State to which the workers concerned are posted and, therefore, it is binding on its institutions.

Accordingly, legislation of the Member State of posting which entitles its competent authorities unilaterally to make a worker subject to the legislation of that Member State on social security matters is contrary to the principle laid down in Article 11(1) of Regulation No 883/2004 that workers are to be covered by only one social security scheme and to the principle of legal certainty which requires, inter alia, that rules of law must be clear, precise and predictable as regards their effects, in particular if they may have negative consequences for individuals and undertakings.

Such legislation is incompatible also with the provisions of Regulations No 883/2004 and No 987/2009 governing the procedure to be followed in the event of difficulties in the interpretation or application of those regulations, in particular when the Member State to which a worker is posted considers that the conditions for the application of Article 12(1) of Regulation No 883/2004 are not met.

As regards the possibility to disregard an A 1 certificate obtained fraudulently, when, in the dialogue provided for in Article 76(6) of Regulation No 883/2004, the institution of the Member State to which the workers have been posted puts before the institution that issued the A 1 certificates concrete evidence that suggests that those certificates were obtained fraudulently, it is the duty of the latter institution, by virtue of the principle of sincere cooperation, to review, in the light of that evidence, the grounds for the issue of those certificates and, where appropriate, to withdraw them.

If the latter institution fails to carry out such a review within a reasonable period of time, it must be possible for that evidence to be relied on in judicial proceedings, in order to satisfy the court of the Member State to which the workers have been posted that the certificates should be disregarded.

However, the abovementioned legislation of the Member State of posting does not satisfy those conditions.

First, that legislation does not lay down an obligation to initiate the dialogue and conciliation procedure provided for by Regulations No 883/2004 and No 987/2009. Secondly, the legislation is not limited to conferring on the national court alone the power to make a finding of fraud and to disregard, on that ground, an A1 certificate, but provides that, outside any court proceedings, national social security institutions and national social security inspectors may decide that posted workers are to be subject to the legislation of that Member State on social security.

(see paras 93-96, 100, 101, 104, 105)