Ex-London Mayor Boris Johnson has compared the European Union's aims to those of Adolf Hitler, arguing that the 28-nation bloc is creating a superstate that mirrors the attempt of the Nazi leader to dominate the European continent.

Johnson's remarks in The Sunday Telegraph elicited outrage on the part of those campaigning to remain in the EU ahead of a June 23 vote on whether to stay or go.

In the interview, Johnson, the most prominent political figure arguing that Britain should leave the EU — opting for "Brexit" — says the past 2,000 years of European history had been dominated by doomed attempts to unify the continent.

He said while EU officials are using "different methods" from the Nazi regime, they share the aim of unifying Europe under one "authority."

He was quoted as saying "Napoleon, Hitler, various people tried this out, and it ends tragically. The EU is an attempt to do this by different methods."

Labour MP Yvette Cooper, a member of the Britain Stronger In Europe campaign, condemned Johnson's comments and accused him of playing a "nasty, nasty game."

Meanwhile, presumptive U.S. Republican presidential nominee Donald Trump is reassuring Britain that leaving the EU will make no difference to any trade deals if he becomes president.

"You would certainly not be back of the queue, that I can tell you," Trump said in an interview with Piers Morgan, host of ITV's Good Morning Britain, to air on Monday.

Last month, U.S. President Barack Obama urged the U.K. to vote for the status quo.

He said Britain would not necessarily get a quick trade deal on its own should it decide to leave the EU, because the U.S. would be focusing on "negotiating with a big bloc, the European Union, to get a trade agreement done."