Boris Johnson “crossed the boundaries” when he made a “dangerous” comparison between Brussels and Adolf Hitler, the European Council president has said.

Donald Tusk 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.

During a press conference in Copenhagen, Mr Tusk told reporters he could not “remain silent” in the face of comments by such an “influential” politician.

He said: “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.”