The arguments sparked allegations of rhetorical excess from both sides and prompted European Council President Donald Tusk to finally enter the referendum debate, deriding Johnson's comments.

Tusk said that "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 in Copenhagen.

Johnson argued over the weekend that the 28-nation bloc was creating a super-state that mirrors the attempt of the Nazi leader to dominate the European continent.

Britain holds a "leave or stay in" referendum on June 23 on its always cantankerous relationship with the EU, and the campaign has become increasingly edgy.

In a speech Tuesday aimed at bolstering arguments for remaining in the EU, Cameron said "it is worth asking the question: Who would be happy if we left?"

He said Russian President Vladimir Putin "might be happy" and that he suspected IS leader Abu Bakr "al-Baghdadi might be happy."