The EU legislature ruled out the Rome I Regulation having immediate application whereby the future effects of contracts concluded before 17 December 2009 would have been brought within its scope. It follows that any agreement by the contracting parties, after 16 December 2009, to continue performance of a contract concluded previously cannot have the effect of making the Rome I Regulation applicable to that contractual relationship without thwarting the clearly expressed intention of the EU legislature. That choice would be called into question if any, even minor, variation made by the parties, on or after 17 December 2009, to a contract initially concluded before that date were sufficient to bring that contract within the scope of the Rome I Regulation, in disregard of legal certainty. On the other hand, the possibility remains that a contract concluded before 17 December 2009 may be subject, on or after that date, to a variation agreed between the contracting parties of such magnitude that it gives rise not to the mere updating or amendment of the contract but to the creation of a new legal relationship between the contracting parties, so that the initial contract should be regarded as having been replaced by a new contract, concluded on or after that date, for the purposes of Article 28 of the Rome I Regulation.

Article 28 of Regulation No 593/2008 on the law applicable to contractual obligations (Rome I) must be interpreted as meaning that a contractual employment relationship that came into being before 17 December 2009 falls within the scope of the regulation only in so far as that relationship has undergone, as a result of mutual agreement of the contracting parties which has manifested itself on or after that date, a variation of such magnitude that a new employment contract must be regarded as having been concluded on or after that date, a matter which is for the national courts to determine.

Article 9(3) of Regulation No 593/2008 on the law applicable to contractual obligations (Rome I) must be interpreted as precluding overriding mandatory provisions other than those of the State of the forum or of the State where the obligations arising out of the contract have to be or have been performed from being applied, as legal rules, by the court of the forum, but as not precluding it from taking such other overriding mandatory provisions into account as matters of fact in so far as this is provided for by the national law that is applicable to the contract pursuant to the regulation.

The list, in Article 9 of the Rome I Regulation, of the overriding mandatory provisions to which the court of the forum may give effect is exhaustive. Thus, to permit the court of the forum to apply overriding mandatory provisions of the legal order of Member States other than those which are expressly referred to in Article 9(2) and (3) of the Rome I Regulation would be liable to jeopardise full achievement of the regulation’s general objective, which, as stated in recital 16, is legal certainty in the European area of justice.

On the other hand — given that the Rome I Regulation harmonises conflict-of-law rules and not the substantive rules of the law of contract — in so far as the latter provide that the court of the forum is to take into account, as a matter of fact, overriding mandatory provisions of the legal order of a State other than the State of the forum or the State of performance of the contractual obligations, Article 9 of the regulation cannot prevent the court seised from taking that matter of fact into account.

This interpretation is not affected by the principle of sincere cooperation laid down in Article 4(3) TEU. That principle does not authorise a Member State to circumvent the obligations that are imposed upon it by EU law and accordingly is not capable of permitting the national courts to disregard the fact that the list of overriding mandatory provisions to which effect may be given, as set out in Article 9 of the Rome I Regulation, is exhaustive, in order to give effect, as legal rules, to the overriding mandatory provisions of another Member State.

(see paras 46, 49, 52, 54, 55, operative part 2)