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LONDON, May 15 (Reuters) - Boris Johnson, a leading campaigner for Britain to leave the European Union at a June 23 referendum, said in an interview that the EU was following the path of Adolf Hitler and Napoleon by trying to create a European superstate.

The former London mayor, a member of Prime Minister David Cameron's Conservatives, 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. There is no single authority that anybody respects or understands. That is causing this massive democratic void."

While fellow pro-Brexit Conservative colleagues supported Johnson's comments, he drew criticism from the "In" camp.

Hilary Benn, foreign affairs spokesman for the opposition Labour Party, said Johnson had lost his moral compass.

"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," Benn said in a statement.

Johnson, a front-runner to succeed Cameron as leader of the Conservative Party, has emerged as one of the most important voices in the "Out" campaign ahead of the June vote.

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 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, 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, to "take over" the Italian economy and to "destroy" Greece.

(Writing by William Schomberg and Kylie MacLellan; editing by Alistair Bell and Jason Neely)