Boris Johnson faced a furious political backlash after he compared European Union efforts to build a federal super state to Hitler's plans to dominate the continent.
While the former London mayor acknowledged the EU was using "different methods" to the Nazis, Remain campaigners said his incendiary comparison to the Third Reich showed he was unfit for high office.
However pro-Brexit Tories said he was simply stating a "historical fact of life" about the failure of successive attempts over the centuries to establish a "greater Europe".
The latest row erupted after David Cameron was attacked last week by Leave campaigners for suggesting that British withdrawal from the EU could lead to the outbreak of the Third World War.
Mr Johnson – seen as the de facto leader of the Leave campaign – said the past 2,000 years 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 immediately condemned by the Remain campaigners with shadow foreign secretary Hilary Benn branding them "offensive and desperate".
"Leave campaigners have lost the economic argument and now they are losing their moral compass," Mr Benn said.
"After the horror of the Second World War, the EU helped to bring an end to centuries of conflict in Europe and for Boris Johnson to make this comparison is both offensive and desperate."
Field Marshal Lord Bramall, a former head of the Army who took part in the D-Day landings, said Mr Johnson's remarks were "simply laughable".
"I know only too well, this comparison of the EU and Nazi Germany is absurd. Hitler's main aim was to create an empire in the East and violently subjugate Europeans," he said.
Former Liberal Democrat leader Lord Ashdown said: "People are fed up with yet another tuppenny tin-pot imitation Churchill promising to 'fight them on the beaches' while weakening our defences and wrecking our economy."
Labour former cabinet minster Yvette Cooper said Mr Johnson was playing a "nasty, nasty game".
"The more he flails around with this kind of hysterical claim, the more he exposes his shameful lack of judgment, his willingness to play the most divisive cynical politics, and the emptiness of his arguments," she said.
Mr Johnson's comments were, however, defended by the pro-Brexit former cabinet minister Iain Duncan Smith who said he was simply stating a "historical fact of life".
"I think the whole process of trying to drive Europe together by force or by bureaucracy and democratic means ultimately makes problems," he told BBC One's The Andrew Marr Show.
"All he is doing in the interview is he is talking about the trend towards the idea of this kind of concept of some kind of greater Europe, that's all."
Tory MP Jacob Rees-Mogg said that Mr Johnson - a classical scholar - had drawn a "very interesting historic parallel".
"Philip II of Spain, Louis XIV of France, Napoleon and Hitler all wanted to create a single European power," told ITV's Peston On Sunday. "What Boris has said is the EU is following the footsteps of these historic figures but using different means."
Former chancellor Lord Lamont said there had been "fascist theorists" who believed very strongly in a united Europe.
He told Sky News's Murnaghan programme: "I don't think he (Mr Johnson) was saying people who favour the European Union were comparable to Nazis.
"He was simply saying that historically, from the Romans, Charlemagne, Napoleon, there have been all sorts of attempts to dominate Europe and these have all floundered because Europe is not naturally one entity."
