5.6 Invalid legislation and horizontal legal relationships

255. The preceding section concerns the substantive incompatibility of national law with Union law. Another scenario is, however, that a directive that has already been transposed into national law is incompatible with a higher source of Union law, such as a Treaty provision or a general principle – i.e. scenario 21. In such an event, on the basis of Articles 263 and 264 TFEU the Court of Justice has jurisdiction to declare the problematic directive entirely2 or partially3 invalid. The invalidity of a directive that has already been transposed into the law of a Member State is likely to impact the enforceability of the implementation measures. Subsequently, horizontal legal relationships that relied on the applicability of the implementation measures may be affected as well.4 When the Court of Justice deems an instrument of Union law invalid, a number of questions arise that are essential for national courts that have to deliver a ruling in horizontal proceedings in which the private parties had relied on the measures implementing the invalid directive. For example, the question arises whether the invalidity of the directive is immediately effective or if the Court of Justice has adopted a transitional period to suspend the effect of the invalidity. And, irrespective of whether the invalidity has immediate effect or whether a transitional period is adopted, does the invalidity of the directive date back in time and, if so, to which date? In other words, are the consequences of the invalidity of the directive applicable only to new legal relationships, or also to legal relationships that were established prior to the ruling in which the directive was declared invalid? And, lastly, how does the Court’s case law on the invalidity of directives relate to the case law on the disapplication of national provisions that are incompatible with primary Union law? The following sections discuss these questions and the concerns related to them.