The creation of the European External Action Service (EEAS) was one of the last greatest steps in the institutional evolution of the European Union (EU) integration process. It represents the first time that the member states agreed to the creation of an autonomous body at supranational level in the field of Foreign Affairs. This fact is more surprising in the case of the big member states, especially the United Kingdom (UK), as it accounts with a worldwide diplomatic network and it is part of several international organizations. The UK was the most contrary member state to the creation of this supranational diplomatic structure. This paper aims to explain why the UK finally agreed to the creation of this diplomatic body. At the same time, the ongoing Brexit is the first time in history that the EU is taking a step back on its path to integration, which raises many questions both for the UK and for the EU. One of them is its future diplomatic relationship. The paper will be informed by a theoretical framework drawn from intergovernmentalism whilst a set of semi-structured interviews with EU and British policy-makers will provide empirical evidence for the paper.