In an interview with ITV News, Mr Cameron was asked if he thought the current Prime Minister was right to prorogue Parliament for five weeks.
Mr Cameron replied: "I don't. 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 and the majorities there are in Parliament."Mr Cameron said he thinks the 21 Tory MPs who lost the whip after voting against the Government in a key Brexit vote last month should be offered a way back into the fold.
• READ MORE: The Guardian apologises to David Cameron for ‘privileged pain’ commentHe added: "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."
Mr Cameron claims Mr Johnson believed Brexit would be "crushed like a toad under the harrow" before he joined the Vote Leave team during the 2016 referendum.
• READ MORE: David Cameron 'rang European leaders to apologise over Remain failure'He said: "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 added: "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.
"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'd never argued for it before.
"He thought it was going to lose and that's why he made the choice."