The president of the European Council has said the only alternative to the EU is "political chaos" and labeled Boris Johnson's recent comments "absurd", BBC reported.
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 June 23 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."
Tusk said the EU remained a "firewall" against conflict between European countries, saying the "banal" truth was that "the only alternative for the Union is political chaos, the return to national egoism, and in consequence, the triumph of anti-democratic tendencies, which can lead to history repeating itself".