Boris Johnson did not believe in Brexit during the referendum campaign and backed Leave “because it would help his political career”, says David Cameron.

In an extract from his memoir published in the Sunday Times, the former PM also refers to cabinet minister Michael Gove as “a foam-flecked Faragist”.

The pair were “ambassadors for the expert-trashing, truth-twisting age of populism”, Mr Cameron writes.

And he also accuses Mr Gove of being disloyal to himself and Mr Johnson.

Of his former colleague, Mr Cameron writes: “One quality shone through: disloyalty. Disloyalty to me – and, later, disloyalty to Boris.”

The latest revelations come after another extract published on Saturday accused the pair of behaving “appallingly” during the 2016 referendum campaign.

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Mr Cameron called the poll after promising it in the Conservative Party’s election manifesto.

He campaigned for Remain, but lost the vote by 52% to 48%, and resigned as prime minister shortly after.

Mr Cameron writes that when deciding whether to back Leave or Remain in the campaign, Mr Johnson was concerned what the “best outcome” would be for him.

“Whichever senior Tory politician took the lead on the Brexit side – so loaded with images of patriotism, independence and romance – would become the darling of the party,” he says.

“He [Mr Johnson] didn’t want to risk allowing someone else with a high profile – Michael Gove in particular – to win that crown.”

The former Tory leader adds: “The conclusion I am left with is that he [Boris Johnson] risked an outcome he didn’t believe in because it would help his political career.”

He also says during the Leave campaign Mr Johnson, who has repeatedly said the UK must exit the EU on 31 October, privately raised the possibility of holding another referendum after fresh negotiations with the EU.

He criticises Mr Johnson’s use of the Vote Leave campaign bus emblazoned by the much-criticised claim that leaving would mean £350m a week extra for the NHS.

“Boris rode the bus round the country, he left the truth at home,” writes the former prime minister.

And of Mr Gove – a cabinet minister both now and then – he said: “I couldn’t believe what I was seeing.

“Gove, the liberal-minded, carefully-considered Conservative intellectual, had become a foam-flecked Faragist warning that the entire Turkish population was about to come to Britain.”

During the run-up to the EU referendum, Mr Gove claimed Turkey and four other countries could join the EU by 2020, increasing the UK’s population by up to 5.23 million by 2030.

However, it was the behaviour of his then employment minister and current Home Secretary Priti Patel that “shocked” him the most, he says.

“She used every announcement, interview and speech to hammer the government on immigration, even though she was part of that government,” he writes.

“I was stuck though: unable to fire her, because that would make her a Brexit martyr.”

Asked about Mr Cameron’s comments, Ms Patel told the BBC’s Andrew Marr that it had been “a privilege to serve” as a minister in David Cameron’s government and that she “enjoyed working with him”.

But she added: “Obviously the referendum has happened, we’ve all moved on, and the fact of the matter is, we are now working to deliver that referendum mandate.

“There is no point in going over the past.”

The prime minister and Mr Gove are yet to respond to the criticisms of them contained in Mr Cameron’s book.

In an interview with the Times published on Saturday, Mr Cameron said he was “hugely depressed” about the 2016 referendum result and he knew “some people will never forgive me”.

But he defended his decision to call the poll, arguing the issue of the EU “needed to be addressed”.

The prime minister is due to meet European Commission president Jean-Claude Juncker in Luxembourg this week as negotiations aimed at securing a deal continue.

In an interview with the Mail on Sunday, Mr Johnson said he was still hopeful a new deal with the EU could be reached in time for the crucial EU summit on 17 October.

It would take a lot of work, he said, adding: “I think that we will get there.”

He said there was a “real sign of movement” in Berlin, Paris and “most interestingly” in Dublin.

However, if he cannot negotiate a deal, the UK would break out of its “manacles” like cartoon character The Incredible Hulk on Halloween, he said.

“Hulk always escaped, no matter how tightly bound in he seemed to be – and that is the case for this country,” he said. “We will come out on 31 October and we will get it done.”

In the interview, Mr Johnson also repeated his opposition to an election pact with Nigel Farage’s Brexit Party, saying the Conservative party was a “great” and “old” party that did not form electoral pacts with other parties.

Earlier this month, Mr Johnson expelled 21 MPs from the party after they rebelled against him in a bid to prevent a no-deal Brexit.

Asked if any would be allowed to stand as a Conservative at the next election, he did not rule it out but urged people not to underestimate the gravity of what they had done.

“They were effectively handing the initiative to our opponents,” he said. “I just want people to understand why it was necessary to be so strict.”

David Cameron as PM

Mr Cameron became the Conservative Party leader in 2005. Five years later he was voted into Downing Street as the UK’s youngest prime minister in almost 200 years – aged 43.

His six-year tenure – firstly in coalition with the Liberal Democrats and latterly with a majority government – was dominated by his desire to reduce the deficit, and the introduction of austerity measures with his Chancellor George Osborne.

But when he pledged in his party’s 2015 manifesto to hold a referendum on the UK’s membership of the EU, the focus shifted.

Mr Cameron backed Remain during the 2016 campaign and, on the morning of the result after discovering he had lost, he announced he would be stepping down, saying: “I do not think it would be right for me to try to be the captain that steers our country to its next destination.”

The former PM had remained silent until this weekend about both of his successors at the helm of the Tory Party – Theresa May and Boris Johnson.

But his allegedly fractious relationship with Mr Johnson has been well documented since their days together at Oxford University – most notably as members of the infamous Bullingdon Club.