Imperial federation was a movement in the late-nineteenth and early twentieth centuries that sought to turn the British Empire into a global federal state. It focused on the self-governing parts of the Empire that included the United Kingdom and the Dominions of Canada, Australia, South Africa, New Zealand and Newfoundland. The unofficial motto of the Imperial federalists was ‘federate or disintegrate’. Many argue that the European Union of today is facing a similar choice. This article will examine Imperial federalist proposals for constitutional reform in the legislative, executive and judicial spheres. It might be expected that the main centres of opposition to Imperial federalism lay at the peripheries of the Empire in Dominions such as South Africa and Canada. This article argues that successive governments of the United Kingdom, the mother country of the Empire, proved to be the most consistent and determined opponents of Imperial federalist initiatives. This article will examine why the government of the United Kingdom acted as the leading opponent of most federal proposals involving constitutional reform. This analysis will also assess the importance of law and popular perceptions of legal tradition in attacking federal schemes throughout the late-nineteenth and early twentieth centuries.