Boris Johnson has compared European Union efforts to build a superstate to Hitler's attempt to dominate the continent.
While the former London mayor acknowledged the EU was using "different methods" to the Nazis, his incendiary comparison is likely to enrage Remain campaigners.
Johnson - seen as the de facto leader of the Leave campaign - said the past 2,000 years of European history had been dominated by doomed attempts to unify the continent under a single government to recreate the "golden age" of the Romans.
"Napoleon, Hitler, various people tried this out, and it ends tragically. The EU is an attempt to do this by different methods," he said in an interview with The Sunday Telegraph.
"But fundamentally what is lacking is the eternal problem, which is that there is no underlying loyalty to the idea of Europe. There is no single authority that anybody respects or understands. That is causing this massive democratic void."