Former London mayor Boris Johnson said the European Union was trying to unify the bloc under one "authority," as did Nazi leader Adolf Hitler, in an interview with British media.
"Napoleon, Hitler, various people tried this out, and it ends tragically," said the outspoken Conservative politician, who is campaigning for Britain to vote to leave the European Union in a June 23 referendum.
"The EU is an attempt to do this by different methods," he conceded in an interview with the Daily Telegraph published late Saturday.
"Fundamentally what is lacking is the eternal problem, which is that there is no underlying loyalty to the idea of Europe," he said.
"There is no single authority that anybody respects or understands. That is causing this massive democratic void."
The opposing Britain Stronger In Europe campaign was quick to react, with former cabinet minister Yvette Cooper of the Labour Party accusing Johnson of playing a "nasty, nasty game," the Press Association reported.
"The more he flails around with this kind of hysterical claim, the more he exposes his shameful lack of judgement," she was quoted as saying.
"He should not try to play political games with the darkest and most sinister chapter of Europe's history."
The dispute was the latest in a contest which has been seen as a struggle for the leadership of the ruling Conservative Party. Prime Minister David Cameron is likely to come under pressure to step down if the public votes next month to leave the EU.