Former London mayor Boris Johnson has said the European Union is following the path of Adolf Hitler by trying to create a superstate.
While he acknowledged the EU was using "different methods" to the Nazis, his controversial comparison to the Third Reich drew a furious response from Remain campaigners.
Mr 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."
His comments were condemned by the Britain Stronger In Europe campaign, with Labour former cabinet minister Yvette Cooper accusing him of playing a "nasty, nasty game".
"The more he flails around with this kind of hysterical claim, the more he exposes his shameful lack of judgement, his willingness to play the most divisive cynical politics, and the emptiness of his arguments," she said.
The row erupted as UKip leader Nigel Farage backed Mr Johnson to succeed David Cameron if there is a vote to leave the EU in the referendum on 23 June.
Mr Farage described himself as a "Boris fan" and compared him to former US president Ronald Reagan.
In comments clearly designed to provoke tensions within the Conservative Party over the referendum, Mr Farage said he could even envisage a situation in which he could work for Mr Johnson in government.
Mr Cameron has always insisted that he will carry on regardless of the result, but many MPs believe he will have no choice but to resign if the country rejects his call to Remain.
In an interview with The Mail On Sunday, Mr Farage rejected suggestions that Mr Johnson was not serious enough to be prime minister if Mr Cameron was forced to stand down.
"Boris goes on surprising people. They say he can't do this, he's a joker - it's like Ronnie Reagan. Could he do it? Yes. If you'd asked me six months ago I'd have said no but I've changed my mind," he said.
Asked if he would like to work for Mr Johnson if there was a vote for Leave, Mr Farage said: "I love Boris, respect him, admire him; I'm a Boris fan.
"Could I work for him? Yes. Could I see a scenario if he was PM and he asked me to do something? I wouldn't rule it out."
An opinion poll published yesterday suggested that twice the number of voters believed Mr Johnson was more likely to tell the truth about the EU than Mr Cameron.
With less than six weeks to go until the referendum, voters are evenly split between wanting to remain in the EU and preferring to leave, other opinion polls have shown.
British citizens living in Ireland can vote in UK/EU referendum. Register on line today.— Charlie Flanagan (@CharlieFlanagan) May 15, 2016