Differentiated integration and the principle of loyalty

Loyalty, as a structural principle of the EU legal order, regulates and constrains differentiated integration. It is the guiding principle underpinning several Treaty provisions on enhanced cooperation and plays an important role in the implementation of opt-out and opt-in mechanisms in specific policy areas. Drawing inspiration from the case law of the Court of Justice on external relations, the article suggests that loyalty may equally constitute a powerful constraint on the member states’ ability to conclude agreements inter se, an instrument that has been used on several occasions as a further tool of differentiation. Applying the principle of loyalty to inter se agreements also suggests a parallel between this form of differentiated integration and enhanced cooperation: the duty of loyalty justifies recourse to inter se agreements solely when enhanced cooperation does not represent a viable alternative.