Boris Johnson "crossed the boundaries" when he made a "dangerous" comparison between Brussels and Adolf Hitler, the European Council President has said.
Donald Tusk, the former Polish Prime Minister, attacked the former London mayor for "political amnesia" over his controversial suggestion likening the EU to the Nazi dictator's plans for domination of the continent.
It comes after the Chancellor George Osborne endorsed the view of the former military chief Field Marshal Lord Bramall, who described Mr Johnson’s remarks as "simply laughable" and "absurd".
The former Mayor had told the Sunday Telegraph that the EU was an attempt to recreate the Roman Empire’s united Europe. “Napoleon, Hitler, various people tried this out, and it ends tragically,” he told the paper.
“The EU is an attempt to do this by different methods,” he added.
In the extraordinary intervention, during a press conference in Copenhagen, Mr Tusk said he could not "remain silent" in the face of comments by such an "influential" politician.
Mr Tusk added: "When I hear the EU being compared to the plans and projects of Adolf Hitler I cannot remain silent. Such absurd arguments should be completely ignored if they hadn't been formulated by one of the most influential politicians in the ruling party.
"Boris Johnson crossed the boundaries of a rational discourse, demonstrating political amnesia. In some sense, he illustrated a state of mind and emotions of many Europeans, not only from the UK.
"In no way, however, can this be an excuse for this dangerous blackout."
On Monday Mr Johnson was also accused of historical inaccuracy by his London mayoral predecessor Ken Livingstone – who is currently suspended from the Labour Party after becoming embroiled in his own Hitler controversy.
“What I said was perfectly true,” Mr Livingstone told the Evening Standard. “But Boris is a lot better informed about Ancient Greece and Rome than about modern history.
“There was never a plan for a United States of Europe under Hitler. What he wanted was actually a Greater Germany that absorbed neighbouring states, with Britain and France rendered subservient.”
But Mr Johnson defended his comments, saying: “Over the last few thousand years there have been all sorts of attempts in Europe to recreate the dream of the Roman Empire and very often that’s been done by force. The EU is different – it’s tried to do it in a more bureaucratic way.
“The problem is there isn’t a single charismatic authority that anyone feels any loyalty to and it’s completely antidemocratic, that’s the problem.
“This discussion is bedevilled by all sorts of artificial media twit-storms or hysteria of one kind or another. There’s a very good argument against the lack of democracy in the EU.
“Over the last 2000 years people have made repeated attempts to unify Europe by force. The EU is a very different project but it is profoundly antidemocratic.”
Join our new commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies