In the field of EU external relations, the ERTA doctrine, the principle of sincere cooperation and the procedural legal basis of Article 218(9) TFEU operate individually and together to restrict Member State participation in international organisations in favour of the Union. Under the ERTA doctrine, the EU gains an exclusive competence to undertake external action if internal rules may be affected. Elsewhere, the principle of sincere cooperation imposes an obligation on the Member States to abstain from taking action in international organisations and allows the EU to exercise its external competences in international organisations of which it lacks membership through the Member States. The scope of the procedural legal basis of Article 218(9) TFEU, which permits the EU to establish positions in international organisations, also enables Union participation to the detriment of the Member States. Additionally, while Article 218(9) TFEU requires the Council to act, in principle, by qualified majority, the Council and some Member States attempt to navigate the procedural requirements of the provision to allow the Council to act unanimously. These three features of EU law do not simply operate individually but they also interact with one other to further erode the ability of the Member States to participate in international organisations. It is necessary to examine how the ERTA doctrine, the principle of sincere cooperation and the procedural legal basis of Article 218(9) TFEU function, both individually and together, to understand how Union participation in international organisations is enabled and Member State action is excluded.