Former London mayor Boris Johnson has claimed the European Union was behaving like German Nazi dictator Adolf Hitler by trying to create a superstate, in a newspaper interview yesterday.

Johnson is one of the leading figures campaigning for Britain to leave the EU in closely-fought referendum being held on June 23.

His comments drew swift criticism from opponents, who said they were “offensive” and showed a “lack of judgement”.

Johnson said the last two thousand years of European history had featured repeated efforts to bring the continent together under a single government, emulating the Roman empire.

“Napoleon, Hitler, various people tried this out, and it ends tragically. The EU is an attempt to do this by different methods,” Johnson told 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.”

Johnson is a leading member of the ruling Conservative party of Prime Minister David Cameron, who is leading the campaign to keep Britain in the EU.