By Westminster Producer Lewis Denison
ITV News understands that around 30 letters of no confidence may have already been submitted to the PM, around two dozen shy of the 54 needed to trigger a leadership vote.
With the mood of Conservative MPs hardening against Mr Johnson, it is looking increasingly likely that a contest could be triggered after - or maybe even before - the publication of the Sue Gray report into allegations of lockdown breaches.
Conservative MPs from the 2019 general election intake are said to have been meeting to discuss getting rid of Mr Johnson, who has been valued by many of his backbenchers for his ability to win non-traditional Tory seats.
Our Deputy Political Editor Anushka Asthana understands that nervous Tory whips are now trying to dissuade colleagues from submitting the letters, warning them that there is no going back once they have.
She has been told that over 10 MPs of the 2019 intake have already submitted letters to 1922 committee chair Sir Graham Brady.
A Cabinet minister said that the 2019 intake feel less loyal than colleagues who have been in the Commons longer, as they did not socialise with fellow parliamentarians as much due to pandemic curbs.
Also hearing about the growing calls of disquiet, ITV News Political Editor Robert Peston was told that a group of Conservative Red Wall and 2019 MPs have met and reached “a mutual understanding” that Mr Johnson should go.
There is speculation that the required 54 number could be reached at some point this week.
The all important 1922 committee is mulling a proposal to cut the minimum time between votes of no confidence from one year to six months, as ITV News Political Correspondent Daniel Hewitt reports.
The move - which could be voted on as early next week - increases the likelihood of the PM being removed from power, as it opens up the possibility of him facing two votes before the summer.
Amid talk about potential leadership campaigns, the relative quiet of Chancellor Rishi Sunak has spoken volumes, as he is seen as a potential successor to Mr Johnson.
The PM refused several times to rule out resigning in an interview on Tuesday, which was seen as disastrous by many.
On Wednesday, Mr Johnson was laughed at during PMQs as he repeatedly urged MPs to wait for Sue Gray's inquiry.
In a dramatic turn, Conservative former Cabinet minister David Davis urged him to resign over the partygate scandal - the most senior Tory MP to do so.
So, what is the 1922 committee and how does it have so much control over the political fate of the PM?
It's a body that represents backbench Tory MPs - effectively a HR department for the party where they are able to air their grievances.
There are weekly meetings, at which the state of the party may be discussed, and the PM is expected to attend himself at least every quarter, however, he may appear more regularly during moments of political significant - as Mr Johnson did during 2019 Brexit negotiations.
If an MP is fed up with a party leader and wants to remove him - as an increasing number are starting to feel about Mr Johnson - they can anonymously submit a letter of no confidence to the 1922 Committee.
Despite its name, the 1922 Committee was actually set up in April 1923 following an initiative by new Conservative MPs elected at the 1922 General Election to facilitate cooperation within the party.
The chairman of the 1922 Committee, elected by its members, is Sir Graham Brady.
If Tories want to remove their party leader, they must submit a letter to the committee chair outlining their aim.
If 15% of the party - under the current Parliament it would be 54 MPs - submit letters to the committee it will trigger a secret ballot, giving MPs the chance to back or boot the leader.
If more than 50% of Tory MPs then vote to remove him, he will lose his role of party leader and be barred from competing in the forthcoming leadership election.
If they win over half the votes, then they remain party leader and are given a year's immunity from any further confidence votes.
Why are some of the 2019 'red wall' Tory MPs particularly angry at the PM? ITV News Political Editor Robert Peston explains
Tory leadership election
If a party leader loses a confidence vote then MPs from across the party can be nominated as potential replacements.
The 1922 Committee will determine how many nominations an MP will require to appear on the ballot.
If more than two qualify then MPs will vote on their preference, leaving two final candidates who must then appeal to party members for votes before being elected leader.
Will Boris Johnson resign, as several of his MPs have urged, or will he be removed?
The prime minister did not rule out a resignation when apologising for attending the party, instead suggesting MPs should wait for the outcome of an internal investigation before demanding his resignation.
Downing Street has said the investigation, being carried out by civil servant Sue Gray, will be concluded and published as soon as possible.
A resignation from Mr Johnson is unlikely in any case and in the meantime the only option available to dissenting Tories is taking the 1922 Committee route.
Sources suggest to ITV News UK Editor Paul Brand that tensions are likely to spill over if damning conclusions are produced from Ms Gray's Whitehall inquiry.
Who is continuing to defend the PM- and how likely is it that he would win a leadership vote?
Despite his embattled position, many unhappy Tories believe the prime minister would still win a confidence vote, which would automatically give him another year in office.
Some of those who want the PM gone have told Anushka that timing is crucial.
They may be forced to wait for the forthcoming local elections in May before deciding whether to push ahead with bids to remove the PM.
But much depends on the severity of the conclusions of Ms Gray's inquiry, which, according to a source, could be published as early as Friday.
Meanwhile, Mr Johnson has retained the confidence of at least two Cabinet ministers, with both Dominic Raab and Sajid Javid publicly backing the prime minister.
Mr Raab said he thinks Mr Johnson will remain leader of the UK for "many years to come", while Mr Javid said the PM "did the right thing by apologising".
Watch Boris Johnson's apology in full:
Why is the PM coming under so much pressure to resign now?
Throughout the coronavirus pandemic his government has faced multiple scandals - from the free school meals debacle, to the UK's sky high death comparable Covid rate, to the schools exams fiasco - however until now the PM had been able to brush away all criticism. But revelations that he attended a Covid-rule-breaking party at the height of the UK's first lockdown appear to be one step too far for some Tories, despite his apology.
And his explanation that he believed it was a "work event" was not enough to quell MPs' concerns.
That's because they had seen an email sent by the PM's principal private secretary Martin Reynolds, leaked by ITV News, which invited more than 100 staff members to attend the event.
The email - which urged staff to "make the most of the lovely weather" by attending "socially distanced drinks" in the No 10 garden - makes it clear that the party was not a work event.
One MP told Mr Johnson he can either resign "with dignity" or the party's 1922 Committee will be forced to remove him.