Boris Johnson, who is leading the "Out" campaign ahead of Britain's European Union membership referendum, said in an interview that the bloc was following the path of Adolf Hitler and Napoleon by trying to create a European superstate.
Johnson told The Sunday Telegraph newspaper that the EU lacked democracy and a unifying authority and was doomed to fail.
"Napoleon, Hitler, various people tried this out, and it ends tragically," Johnson was quoted as saying in an interview.
"The EU is an attempt to do this by different methods. But fundamentally what is lacking is the eternal problem, which is that there is no underlying loyalty to the idea of Europe," the former London mayor said.
"There is no single authority that anybody respects or understands. That is causing this massive democratic void."
Johnson, who is a front-runner to succeed Prime Minister David Cameron, has emerged as the most important voice in the "Out" camp ahead of the June 23 referendum.
Cameron, who is leading the "In" campaign, has argued that Britain's membership of the EU makes the country more secure, more influential and more prosperous. He also says Britain, which is not part of the single-currency euro zone, will not be dragged into ever closer union among the EU's member states.
But an opinion poll published earlier on Saturday suggested that twice the number of voters believed Johnson was more likely to tell the truth about the EU than 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.
In his interview with The Sunday Telegraph, Johnson said he wanted the British people to be "the heroes of Europe" again, creating echoes of the language used by war-time prime minister Winston Churchill, the newspaper said.
It also quoted him as saying tensions between EU member states had allowed Germany to grow in power within the bloc, "take over" the Italian economy and "destroy" Greece.
Johnson's remarks immediately elicited outrage on the part of those campaigning to remain in the EU ahead of a June 23 vote in Britain on whether to stay or leave. The "stay" camp described Johnson's comments as a desperate effort take the focus off the economic impact such a rupture would create and on to the more populist theme of sovereignty issues.It is the second time in a few weeks that Johnson has attracted criticism over the way he has made the case for Brexit.
He was accused of racism last month after suggesting that U.S. President Barack Obama had removed a bust of Winston Churchill from the Oval Office because of anti-British sentiment linked to his "part-Kenyan" heritage.
He made his claims in an article criticising Obama for supporting Britain's EU membership during a visit to Britain.
While Johnson has often been accused of preferring to use bluster and comedy to make his case rather than facts, he faced a particularly heavy backlash over his latest comments.
Hilary Benn, foreign affairs spokesman for the main opposition Labour party and a supporter of EU membership, called Johnson's Hitler comparison "offensive and desperate".
"Leave campaigners have lost the economic argument and now they are losing their moral compass," said Hilary Benn, a senior member of the Labour Party. "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."
The remarks come after a steady drumbeat of economic warnings about the shock to the nation's economy in the event of a vote to leave. The Bank of England and its respected Gov. Mark Carney cautioned last week that Britain faces a recession amid the uncertainty of a so-called Brexit (British exit from the EU).
By shifting to the sovereignty issue, Johnson is changing tack — and tapping into the deep-seated concerns of many Britons who believe that bureaucrats in Brussels have seized too much control over their everyday lives. By evoking World War II, Johnson has reminded much of the country of its "finest hour" — the moment that the nation acted as the bulwark to halt Nazi tyranny.
Johnson' attention-grabbing tactics have also underscored his ability tap into the popular mood, and his comments Sunday are likely to appeal to the patriotic sentiments of those who have not yet made up their minds on the vote. Johnson's self-deprecating manner and jokey persona often seizes the public's imagination — a factor that makes him a leading contender to succeed fellow Conservative Prime Minister David Cameron, the leader of the "stay" campaign.
Another leading figure in the "out" campaign, the nationalist UKIP party leader Nigel Farage, compared Johnson to former U.S. President Ronald Reagan and rejected suggestions he wasn't serious enough to be Britain's prime minister.
"Boris goes on surprising people," Farage told the Mail on Sunday. "They say he can't do this, he's a joker — it's like Ronnie Reagan. Could he do it? Yes."