The Prime Minister told MPs he had attended the “bring your own booze” gathering in the garden of No.10 on May 20, 2020 – during the UK’s first lockdown – for around 25 minutes to “thank groups of staff”.
Mr Johnson, who made the admission at Prime Minister’s Questions on Wednesday despite having previously denied all knowledge of the party, insisted he believed it had been a “work event” as he acknowledged the public “rage” over the incident.
But the confession sparked renewed calls for Mr Johnson to quit, with Mr Ross, the leader of the Scottish Conservatives, speaking directly with the Prime Minister on Wednesday and telling him to go.
Mr Ross has said he would write to the 1922 Committee, which organises Tory leadership contests, to express no confidence in Mr Johnson’s leadership.
At least four MPs, including senior backbencher William Wragg, and 15 MSPs have called for the Prime Minister to quit.
Mr Ross said: “There was one simple question to answer yesterday, indeed from Monday night when we saw this invitation, which was to more than 100 people asking them to join others in the Downing Street garden and bring their own booze.
“If the Prime Minister was there, and he accepted today that he was, then I felt he could not continue.
“What we also heard from the Prime Minister today was an apology and he said with hindsight he would have done things differently, which for me is an acceptance from the Prime Minister that it was wrong. And therefore I don’t want to be in this position, but I am in this position now, where I don’t think he can continue as leader of the Conservatives.
“I spoke to the Prime Minister this afternoon and I set down my reasons and I explained to him my position.”
At least 54 backbench Conservative MPs are required to send letters to the 1922 committee to trigger a leadership challenge.
Tory MSPs responded by rushing to support their leader. Senior members of the party Murdo Fraser and Liz Smith backed the position of Mr Ross, who just last year led the Scottish Conservatives to their joint best result at a Holyrood election and represents the best hope to stopping Scottish independence.
Mr Ross' supporters also included former leader Jackson Carlaw, who held the job on the day of the No. 10 event and, on Twitter, said Mr Johnson had "lost the confidence of the country".
Junior members of the party, including Finlay Carson, Tess White, Russell Findlay, and Megan Gallacher, all publicly backed the move from Mr Ross.
One MSP told The Scotsman that while it was being described as a "brave" move from Mr Ross, it was ultimately the "right call".
Another Scottish Tory MSP said he was more sceptical of the position taken by Mr Ross, but added that public confidence in Covid restrictions was more important.
They said: "In politics you live by the sword and die by it, and no one does politics quite like Boris. But the public mood and anger is palpable, and the buck rests at the top.”
But senior UK Tory minister Michael Gove later appeared to take a swipe at Mr Ross during a meeting of the backbench 1922 committee.
He was quoted telling MPs: "My instant response is he's in Elgin and the national Tory leader is in London."
In the Commons, Mr Johnson earlier said he attended the May 2020 gathering, but “with hindsight I should have sent everyone back inside”.
“I know the rage they feel with me and with the Government I lead when they think in Downing Street itself the rules are not being properly followed by the people who make the rules,” he said.
“And though I cannot anticipate the conclusions of the current inquiry, I have learned enough to know there were things we simply did not get right and I must take responsibility.
"With hindsight I should have sent everyone back inside. I should have found some other way to thank them.
“I should have recognised that even if it could be said technically to fall within the guidance, there are millions and millions of people who simply would not see it that way, people who have suffered terribly, people who were forbidden for meeting loved ones at all inside or outside, and to them and to this House I offer my heartfelt apologies.”
Rishi Sunak was not in the Commons for the apology, with the Chancellor understood to be in Devon amid talk of a leadership bid.
Mr Johnson’s press secretary later insisted the Prime Minister was not a liar and “he is not resigning”.
The secretary claimed the Prime Minister was not among those emailed about the party, but would not explain how Mr Johnson found out about the event.
They also refused to answer if Mr Johnson brought a bottle, if he had a drink, how often he drinks at work or if his then fiancee Carrie Johnson or other ministers attended.
Senior Tory MP Sir Roger Gale said it was already clear that Mr Johnson had to go.
He said: “We now know that the Prime Minister spent 25 minutes at what was quite clearly a party.
“After Prime Minister’s Question time today, it sounds to me, I am afraid, very much as though politically the Prime Minister is a dead man walking."
Mr Wragg, chairman of the Public Affairs and Constitutional Affairs Committee, told BBC Radio 4’s PM programme: “The Prime Minister’s position is untenable and I don’t believe 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.
“I think it is for the Conservative Party – if not the Prime Minister in fact – to make that decision.”
Labour’s shadow Scotland secretary Ian Murray said: “The chamber was angry today and the Tory backbenches crestfallen. They seem to have a mood that they know it’s over.”
Lib Democrats MP Alistair Carmichael claimed he had “never known a mood quite like it”.
Cabinet minister Simon Hart acknowledged the saga was damaging the party and the running of Government.
“There’s a lot of frustration and bafflement about all of this, and I completely understand,” the Welsh secretary said.
“I’ve not met anybody who is not deeply, deeply conscious of the effect that this has on people’s attitudes to not just the political party … but the Government and the smooth running of the nation.”
Labour leader Sir Keir Starmer called on Mr Johnson to resign during Prime Minister’s Questions.
“After months of deceit and deception, the pathetic spectacle of a man who has run out of road,” Sir Keir said.
“His defence … that he didn’t realise he was at a party is so ridiculous that it’s actually offensive to the British public.”
He added: “The party’s over, Prime Minister. The only question is will the British public kick him out, will his party kick him out, or will he do the decent thing and resign?”
The embattled Prime Minister also faced calls to quit from the SNP’s Westminster leader Ian Blackford, who said if he “has no sense of shame, then the Tory backbenchers must act to remove him”.
Liberal Democrat leader Sir Ed Davey said: “”Will the Prime Minister, for the good of the country, accept that the party is over and decide to resign?”
Labour’s Chris Bryant, chairman of the Commons Standards Committee, questioned the Prime Minister’s excuse that he “didn’t spot that he was at a social event”, adding: “How stupid does the Prime Minister think that British people are?.”