The ex-PM said he 'obviously disagreed' with Mr Johnson kicking out the MPs who voted for a law to block a No Deal Brexit.
Urging Mr Johnson to let the rebels back in, he said the decision would be 'disastrous' if it wasn't reversed. He also criticised Mr Johnson's decision to prorogue Parliament, accusing him of 'sharp practice'.
David Cameron (pictured in an ITV interview which aired last night) took aim at Boris Johnson over the expulsion of 21 Conservative rebels
The Tory rebels had the whip withdrawn after they voted with opposition parties to take control of Parliament and pass the law to stop No Deal. As well as former chancellors Philip Hammond and Ken Clarke, they included Churchill's grandson, Sir Nicholas Soames, David Gauke and several ex-ministers.
'I obviously disagree with the idea of taking away the whip from 21 hard-working, loyal Conservatives,' Mr Cameron said. '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.'
The Tory rebels lost the party whip after Boris Johnson (pictured on his eventful visit to Luxembourg yesterday) lost a key Commons vote
At the weekend it was claimed Mr Cameron could even campaign personally for some of the rebels if they stood at the next election, even if it meant campaigning against other Tories.
Asked if he thought Mr Johnson was right to suspend Parliament, Mr Cameron said bluntly: 'I don't.'
He called the decision to prorogue – which No10 claims is for a Queen's speech – 'sharp practice' and designed to 'restrict the debate,' although he said it was not illegal.
'We'll wait for what the courts say. 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.'
Defending his cuts to public spending, he said the strategy 'did work', but suggested he should have cut harder, faster because he was given a 'window of permission' by the public after the 2010 election. 'I think, look, the cuts were very difficult to make and there were lots of very difficult decisions and I'm not sure we got all of them right, but I've never wavered in the belief that it was necessary to make difficult decisions,' he said.
He also revealed that Mr Johnson texted him to say he would back Leave – and predicted the pro-Brexit campaign would be 'crushed like a toad under the harrow'.
Happier times: Mr Cameron (as Prime Minister) and Mr Johnson (as Mayor of London) outside the door of Number 10 in April 2012
During the interview, Mr Cameron condemned Mr Johnson's 'sharp practice' of suspending Parliament in the run-up to the Brexit date
'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. The former prime minister, who left office within weeks of the June 2016 poll, insisted he would never return to frontline politics.
'I love this country. I care passionately about what happens. But I think the idea of going back to frontline politics is not going to happen, nor should it', he said. Mr Cameron also criticised Barack Obama for taking four days to return his calls over President Assad's 2013 chemical weapons attack against his own people.
Mr Cameron said the sight of bodies of children 'laid out in rows' made him think of his late son Ivan, 'and I thought it was just so appalling'.
'I felt we've got to act. President Obama and I had discussed the red line ... of the use of chemical weapons and so I immediately thought; well we must get together and act.'
He confirmed the four-day delay then added: 'I think a more automatic response would have been better. As it was; it took four days to speak. We then agreed a plan.'
Mr Cameron lost the vote in the Commons against intervening to bomb Assad in one of the defining moments of his premiership.
'I blame the people who voted against me, obviously, but I also blame myself. I misread the situation. I'm not saying it would have solved the Syrian crisis, but it was a red line crossed. It was an appalling thing to happen,' he said.