This chapter aims to examine a topic that up to now has received scant, if any, attention: the use of customary international law (CIL) by the European Parliament, the Council and the European Commission. To that effect, the actions resulting from the exercise of different powers of those three institutions (such as proposals, acts of secondary law, parliamentary questions, statements before the CJEU and other courts) have been taken in consideration. The examination of the practice of the political institutions of the EU does not reveal significant differences between them. References to rules of CIL from their side are relatively rare and mostly included in instruments with an international dimension. They all tend to invoke precise rules of CIL when doing so is essential for substantiating their legal position. They also seem to be aware of the fundamentals of this source of international law. By contrast, the European Parliament, the Council and the Commission fail to demonstrate (here is another common feature) that the rules of CIL they invoke are well established or that practice and opinio juris, as constitutive elements of CIL, concur. It must be highlighted as well that no pattern has been identified in this empirical analysis on the use of CIL by the political institutions of the EU.

Much of the debate about the EU and customary international law addresses the conditions under which customary international law is applied within the EU legal order, such as when it is used to challenge the validity of EU acts. To what extent can the EU also contribute to the development and identification of customary international law? This chapter argues that the EU is not only bound by customary international law, but can also contribute to the development and identification of customary law in its own right. It examines these questions in light of the International Law Commission’s 2018 Draft Conclusions on Identification of Customary International Law. It first discusses some of the conceptual issues faced by the ILC and some of the ways that the EU may contribute to the development of customary international law through its practice and opinio juris. The chapter then turns to the role of the Court of Justice of the European Union and the role it plays in the development and identification of customary international law. It argues that EU practice is relevant when determining rules that apply to the relationship between the EU and third states and organizations.

This chapter discusses how customary international law enters the sphere of EU law and to what extent it determines the relationship between the EU institutions, its member states and individual persons. Against this background, it is examined how customary international law is integrated into the EU legal order, what status it enjoys within that legal order, how it can be applied by EU institutions and whether it can serve as a benchmark for judicial review. Assuming that EU law is an autonomous domestic legal system, these questions are discussed from the perspective of EU constitutional law. Here it is argued that the Court of Justice of the European Union, in evaluating these issues, focusses too much on the idea of the autonomy of EU law. In order to provide provisions of customary international law with practical effect the validity of EU law must also be able to be reviewed on the basis of international law standards.

This book covers two classic yet timely issues that have long captured the imagination of international lawyers and EU lawyers alike. The first is on what basis and to what extent customary international law (CIL) applies to international organizations (IOs) like the EU, and how they can shape it. The second is how international custom is perceived, received and incorporated into the legal system of what is undoubtedly the most developed and impactful regional IO in the history of international relations.

The EU functions as a receptor and promoter of customary international law (CIL). However, from a theoretical (and international law) perspective, the questions why and when the EU is bound by CIL still have not been answered in definite terms. This chapter argues that the EU should be afforded a similar treatment as is afforded to (newly independent) states. This is based on a close examination of its practice in the field of international humanitarian law (IHL), where the primary organization tasked with ensuring peace and security – the United Nations – has so far avoided explicitly affirming that it is bound by customary IHL. After showing that it is indeed the EU which might bear direct responsibility in the context of military operations authorized under the Common Security and Defence Policy, the chapter examines the EU’s behaviour when it comes to new regulatory efforts, its executive and operational conduct, and the CJEU’s case law. On the basis thereof, it is concluded that the EU’s practice confirms the theoretical conclusions reached, and that therefore any theoretical uncertainties on whether and why international organizations are bound by CIL might in future be answered by paying closer attention to their practice.

The present chapter examines whether, and if so to what extent, customary international law applies within the internal legal sphere of the EU, that is, between EU member states within the EU law’s scope of application as well as in their legal relations to the EU. In essence, the relevance of customary international law within the EU’s internal legal sphere is about the EU’s assertion of autonomy and self-containment that has been unfurled by the CJEU. The analyses of key areas of customary law (e.g. diplomatic relations, sovereign immunity and equality, rules of responsibility) reveal a complex picture of the its rules’ relevance within the EU. They play a more tangible role in the relationship between EU member states, first and foremost in sovereignty-related areas, than in member states’ legal relationships to the EU. Nevertheless, the present chapter shows that despite being a ‘new legal order’, the EU treaties still constitute a subsystem of public international law, albeit one which manifests typical characteristics of self-containment.