Boris Johnson has dug his own “political grave” following his remarks comparing Theresa May’s Brexit plan to a suicide vest, his former close aide has declared.

Guto Harri, Johnson’s Communications Director during his first term as Mayor of London, broke his silence to launch a withering assessment of his former boss’s conduct.

In an interview for BBC Radio 4’s The Week In Westminster, Harri said that the former Foreign Secretary had ‘blown up’ his own chances of becoming Prime Minister by using inflammatory language and being “sexually incontinent”.

However, in a surprise move, former Cabinet minister Michael Heseltine said Johnson still looks likely to become the next Tory party leader despite recent controversies over his inflammatory language and his private life.

Also speaking to the Radio 4 programme, to be aired on Saturday, Lord Heseltine said it was “difficult” to imagine Johnson not being put on the leadership ballot paper by Tory MPs.

Heseltine, an ardent backer of the UK remaining in the EU, stressed that he still had “substantial” doubts about Johnson’s ability to unify his party and the country but hoped he could change his mind.

The Uxbridge MP has in recent weeks attracted huge controversy after comparing women in burqas to ‘letterboxes’, his ‘suicide vest’ remark and news of his divorce on grounds of adultery.

Harri, Johnson’s right hand man for four years at City Hall, has until now kept his counsel. But he was scathing about Johnson’s recent conduct.

“I fear that Boris is digging. Somebody needs to take the spade out of his hand or it looks to me like he’s digging his political grave,” he said.

“It’s one thing to deploy humour and charm and intellect and all these things he has in spades which he has done brilliantly in the past, not least his exquisite gift of language.

“But at the moment it is being deployed in a really destructive and self-destructive way that I think is doing enormous damage to him as well as to the country.”

The former comms chief, a former BBC political journalist who has known Johnson since they were at Oxford together, added that it was a ‘tragedy’ that his old friend had lurched rightwards.

“People always said he shot from the hip and used language loosely or gaffed. It was always very calculated. It was just calculated in a very very clever way in the past.

“Over a period of time, Boris did move from celebrity to statesman and did widen his appeal enormously.

“He was a huge unifying figure by the end of my time with him when the Olympics happened in London. There were people on left and right. He would not have been re-elected  in a left leaning city like London if he did not appeal to the left.

“Now he’s gone the other way. He’s become more tribal, and tribal within the tribe, so that he would now be – if he were to become leader – a hugely divisive figure.

“Unfortunately he is now dragging us into a place where we think that we can joke about suicide vests and that we can be sexually incontinent.”

Another Boris ally, his biographer Andrew Gimson, defended Johnson, arguing he was a ‘disrupter’ and “what a lot of people who live a long way from London want”.

Gimson argued that the “man in the pub” would not have been moved by the furore over the suicide vest remarks.

But Harri hit back: “There are lots of men in pubs who have been in Afghanistan and Iraq and they can joke about a lot of things.

“But I think to talk about a suicide vest… The problem with the suicide vest for me is this is not Barnier holding the trigger. The very nature of a suicide vest is you blow yourself up. Which is why it is an appropriate analogy for Boris.

“For me he’s misdirected the analogy by saying the trigger is in someone else’s hands. It’s in his.”

Heseltine, who famously launched his own ill-fated leadership bid against Margaret Thatcher, was asked if Johnson had made a fatal error in the way he had approached the party leadership.

“Has he done himself any irreparable harm? Well I don’t think he has,” the former Deputy Prime Minister said.

“What you have to say to yourself is who the Tory Party membership of the House of Commons is going to choose to send to the activists of the Conservative Party in any leadership campaign?”

Under Tory party rules, MPs choose the final two contenders before the election is opened up to the rank and file membership for a ballot.

“Whilst there is strong opposition to Boris, I find it difficult to think of two names that they will send that don’t include him,” Heseltine said. “And, if he gets before the activists, my guess is that he will get the nomination.”

But the Tory grandee added that unless Johnson changed his stance on Brexit he would never unite the country.

“All that is one thing. But if you then ask a second question. Does that unify the party? Does that solve Brexit? Does it get Britain back into the centre ground. Those are the key questions about achieving power and my doubts and reservations are very substantial.”

Johnson pulled out of the Tory leadership race in 2016 after his erstwhile supporter Michael Gove claimed he was not up to the job of PM, and ran himself.

Harri said that moment, as well as Johnson’s miscalculation that the Leave vote would win the EU referendum, “raises questions about his political courage as well as his integrity”.

He added that if Johnson had backed the Remain campaign he would have positioned himself to be David Cameron’s successor.

“His sheer impatience meant that he repositioned himself as something he was not and through that destroyed his own prospects as well as damaged this country.

“The tragedy for me is he could have been Prime Minister and would have been a good one. But now he’s a very different prospect.”

The Week in Westminster, presented by Paul Waugh, is on air at 11am on BBC Radio 4, on Saturday, September 15.