Recent approaches to contemporary Euroscepticism have explained it in terms of the politics of opposition and peripherality characteristic of competitive party systems. Euroscepticism becomes a central strategy by which non-mainstream parties or factions within mainstream parties attempt to gain political advantage. In the British case, there has been a focus on the influence Eurosceptic factionalism can have within a first-past-the-post parliamentary system. This article challenges explanations of British Euroscepticism in terms of the politics of opposition and the workings of the party system. Instead, it is proposed that a structural crisis of British party politics has allowed Euroscepticism to enter the political mainstream. The author conceives of Euroscepticism as a distinct and powerful national movement asserting conceptions of Britain's exceptional national identity. This is viewed as part of a post-imperial crisis that shifts parties, and factions within parties, towards populist forms of legitimation that have weakened possibilities for stable and coherent political leadership over European integration. Consequently, mainstream parties have struggled to protect themselves against the ideological influence of this populist Euroscepticism. This is particularly evident during periods of Eurosceptic mobilization, and is demonstrated in this article through the examination of the extensive role played by right-wing Eurosceptic forces during the attempt by the Major Government to ratify the Maastricht Treaty.