Former deputy prime ministerMr Heseltine described the comments as reckless and irresponsible, while Mr Johnson’s fellow Leave campaigner Chris Grayling repeatedly declined to endorse them in a radio interview.
The Commons Leader said Mr Johnson is a “historian and he was making a historian’s comment” when he likened the EU to the Nazi dictator’s plans for domination of the continent.
But asked seven times if the former London mayor had been right to draw the comparison, Mr Grayling failed to back the comments.
He told BBC Radio 4’s Today programme: “What Boris was talking about was the reality that there is a drive towards greater political integration.
"Boris was making an historical analogy from a historian talking about a whole range of actions since the Roman Empire. He is a historian making a comment in his own words.
"My view is that we should be most concerned about integration in the European Union.”
Mr Heseltine accused Mr Johnson of behaving “irresponsibly and recklessly” and suggested “his judgment is going”.
Asked if he believed Mr Johnson would ever lead the party, he told the BBC: “I’d be very surprised. I think every time he makes one of these extraordinary utterances, people in the Conservative Party will question whether he now has the judgment for that position.”
Asked about Mr Heseltine attacks, Mr Johnson said: “The most important thing is that everybody should cut out the synthetic outrage about things I haven’t said and stick to the facts.”