Since its accession to the European Union, the United Kingdom has played an important role in the design and development of the European Union’s foreign, security and defence policy. While it is among the founding members of North Atlantic Treaty Organization, it is also one of the main contributors to European security and played an active part in developing the relationship between both organisations. With the United Kingdom’s decision to leave the European Union, questions concerning the implications of Brexit on European Union–North Atlantic Treaty Organization cooperation arise. As the transatlantic bridge between the two organisations, Britain also faces an uncertain position within the European security architecture. It therefore needs to redefine its relations with the European Union and its own position among other member states. Taking into account the development of national security interests and recent political events, this article develops three possible scenarios that may occur for the European Union–North Atlantic Treaty Organization relationship depending on the outcome of the Brexit negotiations.