Just hours after Mr Johnson’s apology to MPs, Mr Ross said his position was “no longer tenable”, saying: “I spoke to the prime minister this afternoon and I set out my reasons an I explained to him my position”.
Mr Ross, who is also an MP at Westminster, added he would write to the 1922 committee of backbench Conservative MPs to register his lack of confidence in the prime minister, according to the BBC.
“He is the prime minister, it is his government that put these rules in place, and he has to be held to account for his actions,” the Scottish Tory chief added.
“I don’t think he can continue as leader of the Conservatives.”
His call was echoed by William Wragg, a Tory MP who chairs the Public Affairs and Constitutional Affairs Committee, who said the prime minister’s position was now “untenable”.
“A series of unforced errors are deeply damaging to the perception of the party,” he told BBC Radio 4’s PM programme.
“I don’t think it should be left to the findings of a civil servant to determine the future of the prime minister and indeed who governs this country.”
Earlier, veteran Tory backbencher Sir Roger Gale also told the broadcaster: “Politically, the prime minister is a dead man walking.”
“We know that the prime minister spent 25 minutes at what was quite clearly a party,” he added. “That means that he misled the House”.
While Mr Johnson’s apology to the House of Commons appears to have won him time, Conservative MPs at Westminster remained deeply unhappy with his handling of the party scandal.
Several made clear they believe that an adverse finding from Whitehall mandarin Sue Gray could cost him his job. Her report into the row could be published as early as next week.
One senior Tory told The Independent that MPs in double figures have now submitted letters of no confidence in the prime minister to the chair of the 1922 Committee Sir Graham Brady - with some even going in this afternoon following Mr Johnson’s apology.
Sir Graham needs 54 letters before ordering a vote of no confidence in which a majority of Tory MPs could oust the prime minister.
Several MPs said that they did not expect the threshold to be reached before the publication of Sue Gray’s report, with many waiting to see the findings of the Whitehall mandarin before deciding whether to act.
The executive of the 1922 Committee - an influential body representing backbench Tories - is understood to be meeting this afternoon.
One former minister told The Independent: “There is immense concern, and frankly the excuse doesn’t get anywhere near washing. There are a lot of meetings going on discussing what to do about this and when.”
Sir Roger Gale said: “The prime minister has said what he has said at the despatch box: he spent 25 minutes at what he described as a work event,” said Sir Roger.
“Well, I’m sorry, you don’t have ‘bring a bottle’ work events in Downing Street, so far as I’m aware. And you don’t have ‘bring a bottle’ work events that are advertised or invited by the prime minister’s private secretary.
“The prime minister said on 8 December from the despatch box that he was reliably assured that there were no parties – well, we now know there was at least one party and probably more, and that at least one of them, the one he spent at least 25 minutes at, he attended.
“I think the time has come for either the prime minister to go with dignity as his choice, or for the 1922 Committee to intervene.”
Welsh secretary Simon Hart acknowledged the “frustration and hurt” felt by voters over the reports of parties at No 10.
“I don’t live on a different planet,” the cabinet minister told Welsh journalists in a regular briefing.
“The frustration and the hurt and indignation and the incredulity that emerging stories like this produce. I’ve got, like everyone, family and friends asking me these questions. We have to get to the bottom of this.”
Asked if he could still trust the PM, Mr Hart said: “I know - because I know the guy - he is as anxious as anyone to have this resolved.
“I’ve got to know him sufficiently well to be able to say with confidence I trust him. He is the leader of the government, I’m a member of that and as far as I’m concerned he’s in a very similar position to me in wanting this to be resolved and the facts to be out there and people to make judgements we can all rely on.”
Former minister Dan Poulter said he was “pleased the prime minister has apologised” but said Mr Johnson’s position would be “untenable” if he was found to have misled parliament.
Dr Poulter told The Independent: “It is not much consolation to those of us who cared for patients on the frontline of the NHS and saw them die of Covid.
“I hope the inquiry is conducted very quickly and should the PM be found to have actively mislead parliament or if he faces criminal sanction - or both - then his position would be untenable.”
Tory backbencher Bob Blackman, who yesterday said people were “rightly furious” over the alleged parties, told The Independent: “The apology is welcome but we will have to see what the outcome of the investigation is.”
The Harrow East MP declined to say whether Mr Johnson would have to go if the Gray report found against him: “There’s no point having a report if you are going to pre-empt what it says.”
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