There are claims that letters from MPs demanding a vote of no confidence - 54 would trigger a challenge - were being submitted to Sir Graham Brady, who chairs the backbench 1922 Committee
Boris Johnson is battling to survive as prime minister, despite throwing himself on the mercy of MPs and apologising for the "bring your own booze" Downing Street party, SkyNews said.
With top Tories joining opposition leaders in calling for him to resign, he faces a perilous few weeks that could see his two-and-a-half-year premiership brought to a dramatic end.
Tory MPs were incensed by the PM's claim that the party on 20 May 2020 was a work event that did not break lockdown rules, and claimed a leadership challenge was now only a matter of time.
There were claims that letters from MPs demanding a vote of no confidence - 54 would trigger a challenge - were being submitted to Sir Graham Brady, who chairs the backbench 1922 Committee.
Poll makes grim reading for the Conservatives
And a new opinion poll showing a massive slump in Tory support and a 10-point lead for Labour - the party's biggest lead since December 2013 - will spread terror through Conservative ranks.
As well as confirming recent polls suggesting six in 10 voters think Mr Johnson should resign, the poll by YouGov for The Times - conducted before the PM's apology - suggests Labour support is 38% (+1), Conservatives 28% (-5) and Lib Dems 13% (+3).
The Times also reports that Tory MPs said the PM told them in the Commons Tea Room after PMQs that he did not believe he had personally done anything wrong, and said: "We're taking hits for something we don't deserve."
The absent chancellor
After a day of electrifying drama at Westminster, all eyes were on Mr Johnson's potential leadership rivals Rishi Sunak and Liz Truss, as MPs speculated whether they were poised to move against the PM.
While most cabinet ministers offered qualified support to Mr Johnson after his "high noon" showdown with MPs at Prime Minister's Questions, it was several hours later before the two front-runners backed him.
The chancellor's motives were questioned earlier when he chose to miss PMQs and instead make a 200-mile trip to Devon. Ms Truss was present alongside Mr Johnson, but had been answering questions immediately before PMQs.
More than eight hours after the PM's apology, at 8.20pm, Mr Sunak eventually broke his silence in a tweet that many MPs will consider lukewarm in his support for the beleaguered prime minister.
"I've been on a visit all day today continuing work on our Plan for Jobs as well as meeting MPs to discuss the energy situation," Mr Sunak said.
And referring to the current Whitehall inquiry into the party allegations, he added: "The PM was right to apologise and I support his request for patience while Sue Gray carries out her enquiry."
An hour later, at 9.14pm, Ms Truss was more fulsome in her support, tweeting: "The Prime Minister is delivering for Britain - from Brexit to our booster programme to economic growth. I stand behind the Prime Minister 100% as he takes our country forward."
Some MPs were comparing Mr Sunak's eight-hour silence to then Chancellor John Major's failure to endorse Margaret Thatcher in the 1990 leadership election, when he claimed he was recovering from a wisdom tooth operation.
It was also pointed out that Tony Blair's allies used to call Labour Chancellor Gordon Brown "Mcavity", after TS Eliot's fictional cat that used to disappear, when Prime Minister Blair was in trouble.
Tory MPs turn on the prime minister
Tory calls for Mr Johnson to quit were led by the party's leader in Scotland, Douglas Ross, and backed by his charismatic and popular predecessor, Ruth Davidson.
Caroline Nokes, who chairs the Women and Equalities select committee, later called for Mr Johnson to resign. "He's damaging us now, he's damaging the entre Conservative brand," she said in a TV interview.
William Wragg, a lockdown critic, chairman of the Public Administration select committee and a vice-chairman of the 1922 Committee, also demanded the Prime Minister's resignation, claiming MPs were "worn out of defending what is invariably indefensible".
Another senior Tory MP, Sir Roger Gale, called Mr Johnson a "dead man walking" and urged the executive of the 1922 Committee, known in political folklore as "the men in grey suits", to tell the PM it was time for him to go.
And in a menacing warning to the PM, leading lockdown rebel Mark Harper served notice that Whitehall enforcer Sue Gray's report could make or break the prime minister.
"When we know all the facts it will be up to every Conservative MP to consider what action needs to be taken to ensure our country has the leadership it deserves," he said in a statement.