The former PM said his successor had never argued in favour of leaving the EU before the referendum campaign in 2016.
He also condemned his 'sharp practice' of suspending Parliament in the run-up to the Brexit date - and warned the Tories face 'disaster' unless Remainer rebels who were brutally ousted from the party are allowed to return.
The outspoken assault on Mr Johnson came in an interview with presenter Tom Bradby ahead of the publication of Mr Cameron's long-awaited memoirs this week.
David Cameron delivered a searing verdict on Boris Johnson in an interview with ITV's Tom Bradby ahead of the publication of his long-awaited memoirs this week
During the interview, Mr Cameron condemned Mr Johnson's 'sharp practice' of suspending Parliament in the run-up to the Brexit date
Mr Cameron sat down with ITV's Tom Bradby (pictured right) for an extended interview
Mr Cameron said he had never heard Mr Johnson argue in favour of cutting ties with Brussels before the referendum.
'I spoke to him at length about it and I said, 'Boris you've never been in favour of leaving the EU, so why now there's a better deal on offer, are you in favour of leaving now,' Mr Cameron said.
'My conclusion is; he thought that the Brexit vote would be lost but he didn't want to give up the chance of being on the romantic, patriotic nationalistic side of Brexit.'
Mr Johnson very publicly wavered between supporting Leave and Remain as the crucial national vote loomed in 2016.
Mr Cameron said: 'Minutes before he went out to explain why he was going to be on the side of Brexit, he sent me a text saying, ''Brexit will be crushed like a toad under the harrow.''
'But I can only conclude that... he thought it was going to lose and that's why he made the choice.'
Mr Cameron was asked about Mr Johnson's tactics in deciding to prorogue Parliament until October 14, in the face of opposition from Remainer MPs.
Scottish judged ruled last week that the move was illegal, although the English courts decided otherwise. The Supreme Court is due to start hearing arguments tomorrow in a titanic clash.
Mr Cameron said: 'I don't think, it was illegal, It looked to me, from the outside, like rather sharp practice of trying to restrict the debate and I thought it was actually from his point of view probably counterproductive.
'In the end, we have to work through parliament, and you can't deny the arithmetic of parliament and the majorities there are in parliament.'
Mr Cameron said he had never heard Mr Johnson (pictured in Luxembourg today) argue in favour of cutting ties with Brussels before the referendum
He also slammed Mr Johnson for expelling 21 MPs from the Parliamentary party after they rebelled to back legislation ruling out No Deal. Amber Rudd later resigned from the Cabinet and gave up the whip in solidarity with the mutineers.
'I obviously disagree with the idea of taking away the whip from 21 hard-working, loyal Conservatives.
'I think that was a bad decision, if it isn't reversed, it will be I think a disastrous decision.
'I hope that Boris will get a deal in Brussels, he will come back, try and bring parliament together to back that deal - I don't see why those 21 people shouldn't be restored to the Conservative whip.
'If they're not, I really worry about what could happen.'