It comes as Mr Johnson admitted at today's Prime Minister's Questions he attended a "bring your own booze" party at Downing Street during the UK's first coronavirus lockdown. After he told MPs the event in the No 10 garden was "technically within the rules" but he should have realised it would look different to the public, he is facing calls to quit – including from Mr Ross and Ms Davidson, of his own party.

       On Tuesday, a visibly furious Mr Ross said that if the Prime Minister was found to have misled the Commons by breaking COVID-10 guidelines, he should resign.

       In an STV interview, he was asked: "If he [Boris Johnson] was at the party he can't continue as Prime Minister, can he?"

       Mr Ross replied: "No – because it's a breach of the guidance. It's a breach of the rules that was put in place. It's also breaking the law.

       "You cannot be the head of the government that is asking people to follow these rules and then break those rules yourself."

       Today, Conservative MPs are being urged to speak out.

       SNP Westminster leader Ian Blackford said they have "a duty" to remove Mr Johnson after the latest party gate revelation and admission.

       Mr Blackford tweeted: "Tory MPs, including Douglas Ross, put Boris Johnson into power. They knew exactly who he was then – and they share the blame for the damage he's doing.

       "As Johnson refuses to resign, Tory MPs have a duty to remove him from office. This is now a question of their own integrity."

       Mr Ross has this afternoon finally broken the ranks. He said on Sky News: "Regrettably, I have to say his position is no longer tenable. I spoke to the Prime Minister this afternoon and I set out my reasons and I explained to him my position.

       He also told reporters: "I said yesterday if the Prime Minister attended this gathering, party, event in Downing Street on May 20 then he could not continue as Prime Minister."

       Ms Davidson, commenting on Mr Ross's demand for Mr Johnson to resign, said on Twitter: "A tough call to make. But the right one."

       The Prime Minister is facing his most serious threat yet after the email from his private secretary Martin Reynolds inviting more than 100 people to the BYOB party gathering emerged.

       The gathering took place on May 20, 2020.

       Last month, when asked about the claims of other Downing Street parties, Mr Johnson told parliament all Covid guidance had been followed, no rules had been broken and no party had taken place in No 10.

       This time he admitted he was there, though not without excuses.

       At PMQs, Johnson told MPs he attended the gathering for around 25 minutes to "thank groups of staff" and said he did not think it was a party.

       He claimed: "I believed implicitly that this was a work event.

       "With hindsight, I should have sent everyone back inside, I should have found some other way to thank them, and I should have recognised that — even if it could have been said technically to fall within the guidance — there would be millions and millions of people who simply would not see it that way."

       Touching upon the Prime Minister's remarks, Mr Ross said: "He believes that he didn't do anything wrong and he has put up a defence of his position.