Her anger was shared by former colleagues at Holyrood, with one telling the Daily Telegraph that Tory MSPs would be “cartwheeling down the Royal Mile” if Mr Johnson was forced to quit.

Another told The Telegraph the furore was the “latest episode in the s—storm that is Boris Johnson”, while another said: “The sooner he goes the better.”

Support on the Westminster Tory back benches at Westminster was said to be evaporating, with one source saying it was “very hard to see the way out.”

“A lot of MPs who are not necessarily against him are fed up with the psychodrama. Even if he gets through this thing, what is the next thing?” the source asked.

They predicted Mr Johnson would resign rather than being forced out, saying he was “conscious of his place in history” and did not want the indignity of being ousted in the manner of Margaret Thatcher or Theresa May.

Mr Ross, who resigned as a Scotland Office Minister in protest at Dominic Cummings’ lockdown trip to Barnard Castle, told STV the public’s fury over the parties was “deserved and understandable.”

Mr Johnson has said the question of whether he attended was a matter for Sue Gray, the senior official charged with investigating the parties. However, Mr Ross said the Prime Minister answering this would “not in any way” undermine the inquiry.

He predicted there was “no way” Mr Johnson would get through Prime Minister’s Questions on Wednesday without stating whether he was there.

Nicola Sturgeon, the Scottish First Minister, urged Mr Johnson to resign, telling MSPs it appeared there was “not just one isolated breach but serial breaches of guidance that people were following”.