Boris Johnson has dismissed criticism of his comparison between Adolf Hitler and the European Union as “synthetic outrage”, after Conservative grandee Lord Heseltine suggested it had damaged his chances of ever leading the party.

Former deputy prime minister Lord 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 Grayling sidestepped questions over whether the Tories were engaged in civil war.

“Yes, we are having a lively debate within the Conservative Party, but we are also getting on with governing the country,” he added.

Lord 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, the peer 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 the former DPM’s attacks, Mr Johnson told TV cameras outside his London home: “The most important thing is that everybody should cut out the synthetic outrage about things I haven’t said and stick to the facts.

“The facts are that the EU is now producing about 60% of the law made in this country, it’s changed out of all recognition from what we signed up to in 1972, it is making it impossible for us to control our borders and it costs about £350 million per week.

“The only safe option is to vote Leave on June 23.”

Mr Johnson has refused to rule out a second Brexit vote if next month’s referendum sees a knife-edge victory for the Remain campaign.

He insisted he expected the Leave side to score a “decisive” victory, but left the door open to pushing for a second poll if things did not go his way.

He said: “I think to start getting into all sorts of complicated, off-the-back-cushion calculations about further referendums is pointless. We are going to win.”

On Tuesday, 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.”