The president of the European Council yesterday said the only alternative to the EU is "political chaos" and labelled Boris Johnson's recent comments "absurd", BBC reports.
Donald Tusk said he was speaking out to defend his "vision of Europe".
He said he could not "remain silent" after Johnson compared the EU's aims with Hitler's, and said the ex-London mayor had shown "political amnesia".
Leave campaigners said EU chiefs joining the UK's referendum debate would help their cause.
The UK votes on 23 June on whether to remain in the EU or to leave.
Tusk chose a joint press conference with the Danish prime minister to make his intervention, saying: "To defend does not mean to lecture anyone.
"The British citizens will make this decision themselves and they do not need any whisperers, especially from Brussels," he said.
"I understand this very well. But 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 of the ruling party."