David Cameron has criticised Boris Johnson for removing the whip from 21 Conservative MPs.
The former Prime Minister was speaking on ITV on Monday night ahead of the release of his autobiography.
Mr Cameron, who was at the University of Oxford with Mr Johnson, also warned that proroguing Parliament was a "mistake".
He said that kicking out the MPs, including Winston Churchill’s grandson, was a "bad decision".
David Cameron - a career in pictures
He added: “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 went on to say that in his opinion the new Prime Minister's support of leave in the 2016 referendum was not genuine.
He said: “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."
And speaking on the subject of the claim that £350 million was sent to Europe, Mr Cameron added: "They got on the bus and they left the truth at home."
Despite admitting that he was to blame for the instability since the referendum in 2016, Mr Cameron maintained that the vote had been "inevitable".
But the 52-year-old said that he had "failed" in his attempt to try and settle issue.
He said: "If you're asking me, do I have regrets? Yes. Am I sorry about the state the country's got into? Yes.
"Do I feel I have some responsibility for that? Yes. It was my referendum, my campaign, my decision to try to renegotiate.
"And I accept all of those things and people, including those watching this programme, will have to decide how much blame to put on me.
“I cannot put it more bluntly than this - I accept that that attempt failed.”
At the end of the interview, Mr Cameron said that he would not be returning to front line politics.
He added that while he texts Mr Johnson advice, the new PM does not always take it.
