Video report by ITV News Political Correspondent Angus Walker
In another stinging attack from his long-awaited memoir, Mr Cameron said Mr Johnson privately suggested there could be a "fresh renegotiation, followed by a second referendum" - which he now says he opposes.
It's the latest volley of comments from the former leader with Mr Cameron also hitting out at Michael Gove, calling him a "foam-flecked Faragist" over his statements on Turkey joining the European Union.
He believes Boris Johnson "wanted to become the darling of the party," and did not "want to risk someone else high profile - Michael Gove in particular - winning that crown".
"The conclusion I am left with is that he risked an outcome he didn't believe in because it would help his political career," Mr Cameron said.
Further revelations include Mr Gove's claim that the public were tired of experts made him "an ambassador for the truth-twisting age of populism".
"By the end, Boris and Michael seemed to me to be different people. Boris had backed something he didn't believe in.
ITV News Presenter Tom Bradby previews interview with David Cameron
"Michael had backed something he did perhaps believe in, but in the process had broken with his friends ... while taking up positions that were completely against his political identity.
"Both then behaved appallingly, attacking their own government, turning a blind eye to their side's unpleasant actions and becoming ambassadors for the expert-trashing, truth-twisting age of populism."
On Mr Gove, the former prime minister said: "One quality shone through: disloyalty. Disloyalty to me and, later, disloyalty to Boris."
"I’m not saying one will happen or should happen. I’m just saying that you can’t rule things out right now because you’ve got to find some way of unblocking the blockage," he said.
He told the paper: “I say in the book: Boris had never argued for leaving the EU, right?
"Michael was a very strong Eurosceptic, but someone whom I’d known as this liberal, compassionate, rational Conservative ended up making arguments about Turkey (joining) and being swamped and what have you.
"They were trashing the government of which they were a part, effectively."
Mr Cameron said attacks by Priti Patel, who is now the Home Secretary, on his government's immigration record "shocked me most" but he did not want to fire her and create a "Brexit martyr".
He also says that Dominic Cummings, who is now a special adviser to the government. was part of a "cauldron of toxicity" with Nigel Farage.
David Cameron's first in depth television interview about his time in office will be broadcast on ITV on Monday 16th September at 8pm