Boris Johnson backed Brexit to "help his political career," David Cameron says.

The former prime minister says Johnson campaigned to leave the EU so he could become the "darling" of the Conservative party — but didn't actually believe in Brexit.

In extracts of his upcoming book published on Sunday, Cameron also accuses Johnson of lying during the referendum campaign.

"Boris rode the bus round the country, he left the truth at home."

Cameron is breaking his silence over three years after leading the unsuccessful Remain campaign and resigning as prime minister.

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Boris Johnson lied during the Brexit campaign and backed leaving the European Union because it would further his career, according to the United Kingdom's former prime minister, David Cameron.

In the latest extracts from his upcoming book published in the Sunday Times newspaper, Cameron says the UK's current prime minister backed Brexit in the 2016 referendum because "it would help his political career."

"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," Cameron says.

"He [Johnson] didn't want to risk allowing someone else with a high profile — Michael Gove [current senior government minister] in particular — to win that crown."

The former prime minister adds: "The conclusion I am left with is that he [Johnson] risked an outcome he didn't believe in because it would help his political career."

Read more: Ex-Conservative minister Sam Gyimah joins the Liberal Democrats, accusing Boris Johnson of embracing "English nationalism"

Cameron called the 2016 referendum and went on to lead the unsuccessful campaign to stay in the EU.

He resigned as prime minister following the Leave vote, and has rarely commented on politics since then. However, his memoirs, which will be published in full next week, will reveal how he feels about the British political crisis.

In the extracts published today, Cameron accuses Johnson of lying while fronting the Leave campaign.

"Boris rode the bus round the country, he left the truth at home," says Cameron, referring to the official Leave campaign's debunked claim that Brexit will create an additional £350 million a week for the National Health Service.

Boris Johnson and Michael Gove. Chris J Ratcliffe/Getty Images

Cameron is particularly scathing about the current Chancellor of the Duchy of Lancaster, Michael Gove.

Gove, who campaigned to leave the EU alongside Johnson, was a "liberal-minded, carefully-considered Conservative intellectual" who became "a foam-flecked Faragist warning that the entire Turkish population was about to come to Britain," Cameron writes, adding: "I couldn't believe what I was seeing."

In other parts of Cameron's book published on Saturday, Cameron accuses senior Leave campaigners of behaving "appallingly" during the Brexit referendum campaign.

He says the result left him "hugely depressed" and that he thinks about it "every single day."

Pro-EU campaigners accused Cameron of refusing to accept responsibility for the Brexit crisis.

"While I completely agree that Boris Johnson has no shame, David Cameron is also guilty of putting party and personal interests over those of the country. We're in this mess because of his bad decision-making," Naomi Smith, CEO of Best For Britain, told Business Insider.

"These comments highlight why Parliament and the country is sceptical of this administration's call for a general election before Brexit. No one can trust them not to pull some kind of trick.

"But, as Cameron admits, there is another way out of the current gridlock. He joins the growing ranks of former prime ministers who see the need for a final say on Brexit."