2.3.1 Consistent interpretation of national law with Union law

21. Both in vertical and horizontal cases that fall within the scope of Union law, for the sake of the effet utile of Union law Member States – and particularly national courts – are obliged to interpret national law as a whole in harmony with Union law.1 This obligation follows from Article 4(3) TEU in conjunction with Article 288 TFEU and is “inherent in the system of the Treaty”.2 By interpreting national law in harmony with Union law, the respective rule of Union law has effect in horizontal legal relationships. Although the obligation upon Member States to interpret national law in line with Union law applies to all sources of Union law, the obligation flourishes in the realm of directives and other rules of Union law that lack direct horizontal effect, like certain Treaty provisions and the European Union Charter of Fundamental Rights.3 By interpreting national law in harmony with these type of provisions of Union law, it is assured that as much effect as possible is given to them via the application of the respective national provisions that apply to the legal relationship in question.

22. In terms of the content and scope of the obligation, the Court of Justice has held that national courts must interpret national law consistently with Union law as far as possible whenever they can possibly do so.4 In relation to directives the duty of consistent interpretation exists from the expiry of the implementation period.5 Not only implementation measures should be interpreted consistently with Union law, but the whole body of domestic law, including open norms in for example the law of obligations or civil liability law.6 Moreover, the Court of Justice held that the obligation to interpret national law in conformity with Union law includes the obligation for national courts to change their national settled case law insofar as it is based on an interpretation of national law that is incompatible with Union law.7 Further, when interpreting national law with Union law, national courts must use all the interpretation methods that are at their disposal in the law of their Member State.8 National courts are thus required to interpret national law, “as far as possible, in the light of the wording and the purpose” of Union law.9

23. Notwithstanding this, the duty of consistent interpretation is restricted by general principles of law, in particular the principle of legal certainty, the prohibition of retroactive effect and the principle of res judicata.10 Also, although the obligation entails the duty of national courts to change their established case law11, the obligation does not reach so far that it requires an interpretation of national law contra legem.12 Nonetheless, if their domestic procedural law allows interpretation contra legem, national courts are held to apply this interpretation all the same.13 In other words; Union law does not oblige – nor prohibit – national courts to interpret national law contra legem; this is a matter governed by national law. In this context it should be noted that national systems may have methods or instruments that in their effect are equal to contra legem interpretation without being labelled as such. For example, in Dutch law a provision of mandatory law can be disapplied if application of that provision would be unacceptable in view of the concrete circumstances of the case.14In the context of their obligation to interpret national law in conformity with Union law, national courts are obliged to explore if such contra legem-alike methods or instruments can indeed be applied. In Member States where interpretation contra legem is not allowed, a national court must conclude that when national law cannot be interpreted in conformity with Union law, the obligation of consistent interpretation does not bring solace to the case.15 Sometimes, however, the national court will have another tool at its disposal: under certain conditions, the national court is obliged to set aside national law as a result of a compatibility review against Union law.