There have been reports that Mr Johnson intends to fight on if he is fined. The Prime Minister has declined to say he would resign if found to have broken Covid laws.

But Sir Iain Duncan Smith, the former Conservative Party leader, said Mr Johnson would struggle to continue in such circumstances.

“That’s a decision made by my colleagues but I think it would be very tough for anyone to remain after that”, Sir Iain told The I newspaper on whether Mr Johnson could remain in post if he was found to have broken the law.

Allies not convinced PM will be fined

One Tory loyalist on Friday signalled there was growing feeling among some of Mr Johnson’s allies that he was unlikely to be given a fixed penalty notice.

“On the basis of all the facts we know, he was always in either his home, his garden, or his workplace during the events being looked at,” the source said.

“The PM has always got a legitimate reason to be there. I think it’s quite hard to fine Boris. It’s highly unlikely that he organised any of this stuff.”

Rishi Sunak, the Chancellor, said on Friday he did not expect to receive a questionnaire from the Met despite being at an event marking Mr Johnson’s birthday party which is being investigated.

Asked by Sky News whether he had received a questionnaire from the Metropolitan Police, Mr Sunak said: “No.”

Asked whether he expected to receive a form to complete, he replied: “No… well, I don’t know.”