His intervention came after Douglas Ross, the Scottish Conservative leader, claimed rebel Tory MPs are getting “closer and closer” to the 54 letters needed to trigger a vote of no confidence in Mr Johnson.

He said the situation was a “rollercoaster ride” as some MPs were withdrawing their letters to the 1922 committee in response to party whips but he still thought the total was “near” the threshold.

Asked by BBC Scotland whether Mr Johnson should resign, Mr Kerr said: “I believe he should. I think the Prime Minister is damaged beyond recovery. And I think that, in due course, that is exactly what’s going to happen. The Prime Minister will go, yes.

“I’ve been a Conservative all of my adult life and I find myself in this very unusual and painful position of actually seeking for a Conservative prime minister to leave office.”

He predicted that the Gray report would prove that “there have been events held in Downing Street over the period of the lockdown which were wholly inappropriate, at least, and at worst probably breaking the very laws that the people in those buildings in Downing Street were making for the rest of us”.

Mr Kerr cited an “extremely damaging” TV interview given by Mr Johnson this week in which he said he was not told that a Downing Street garden party he attended on May 20, 2020, was against government rules banning such social gatherings.

He added: “I’m sorry, it was a very sad interview. I felt sorry for him in the worst possible sense of the meaning of those words.”

Meanwhile, Ms Ross said he had not been threatened by government whips to withdraw his letter of no confidence and had not heard of this happening to others.