The author attempts to answer the following question: in an international legal system of which States are no longer the only subjects, what is the role that the consent of IOs plays in the creation of rights and obligations that apply on the international plane? The chapter reflects on the legal agency that IOs enjoy, qua subjects of international law, in the process of formation of general international law and in the acquisition of treaty rights and obligations. In so doing, it advances two claims. The first is that even though IO practice can and should be taken into account in the identification of customary rules, it is not a necessary element in the process of formation of those rules. The second claim of the chapter is that, under current law, IOs can only be bound by treaty rules to which they have consented, which may give rise to problematic gaps between the treaty obligations of IOs and the treaty obligations of their members. In pursuing those claims, the chapter offers some reflections on the systemic and normative implications of the formal involvement of IOs in the making of rules of general and conventional international law.

The EU is often described as a sui generis international organization. While this is an apt description in some ways, it is far from self-evident that the special features that the EU exhibits are or should be part of the explanation of why and how it is bound by customary international law. This chapter seeks to show that, in the eye of public international law, the EU is not yet in a category of its own which would justify or otherwise affect the applicability of custom to it. It does so by investigating whether a special category of ‘regional economic integration organizations’ (REIOs) has emerged to which international law grants ‘preferential’ treatment, with a focus on three rules that were unsuccessfully proposed to cater for the specificities of the EU in codification projects carried out by the UN International Law Commission. The chapter also considers doctrinal and normative reasons why calls to distinguish the EU from other international organizations under customary international law have fallen on deaf ears, while recognizing the role that the EU has played in widening the horizons of the law of international organizations.

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 Chagos Advisory Opinion is in many ways a bold judgment. For one, the International Court of Justice (ICJ) agreed to give advice to the UN General Assembly (UNGA) against the wishes of three permanent members of the UN Security Council, in circumstances in which it could have dodged the bullet with a reasonable degree of plausibility.1 It declined the invitation to characterise the request as one concerning a bilateral territorial dispute between Mauritius and the UK, and sided, instead, with the several states who viewed decolonisation as a multilateral issue in which the UN retains a legitimate interest. The Opinion, moreover, comprises a scrupulous retelling of the political process leading to the excision of the Chagos Archipelago from Mauritius in the 1960s, and provides a cutting answer to the UNGA’s questions: the process of decolonisation of Mauritius has not yet been completed, and the UK is under an obligation to bring its administration of Chagos to an end.