A major public debate on the costs and benefits of the United Kingdom’s membership of the European Union is presently under way. The outcome of the referendum on 23 June 2016 will be a pivotal moment in determining whether the EU has a future as a component of the UK’s European diplomatic strategy or whether there is a major recalibration of how the UK relates to Europe and more widely of its role within international relations. Since accession to the European Economic Community the UK has evolved an uncodified, multipronged European diplomatic strategy. This has involved the UK seeking to reinforce its approach of shaping the security of the continent, preserving a leading diplomatic role for the UK in managing the international relations of Europe, and to maximize British trade and investment opportunities through a broadening and deepening of Europe as an economically liberal part of the global political economy. Since accession the UK’s European diplomatic strategy has also been to use membership of the EU to facilitate the enhancement of its international influence, primarily as a vehicle for leveraging and amplifying broader national foreign and security policy objectives. The strategy has been consistent irrespective of which party has formed the government in the UK. Increasing domestic political difficulties with the process of European integration have now directly impacted on this European strategy with a referendum commitment. Whether a vote for a Brexit or a Bremain, the UK will be confronted with challenges for its future European strategy.