The PM has faced a steady drip of letters calling for a vote of no confidence - ITV News' Carl Dinnen has the latest.
Boris Johnson has, according to some of his own MPs, been calling those critical of him and hinting at promotions in a bid to shore up support as the number of no confidence letters against him continues to grow.
Another letter was submitted on Tuesday by the Conservative MP for Carlisle after one of the prime minister's former business secretaries accused him of "unacceptable failings of leadership that cannot be tolerated".
Andrea Leadsom, who previously served in the PM's Cabinet, appeared to urge her Tory colleagues to consider Mr Johnson's future when writing a letter to her South Northamptonshire constituents.
She told them it had become "painfully clear" to her after reading Sue Gray's Partygate report that it was "the responsibility of the prime minister" to have stopped severe rule-breaking on Downing Street during the pandemic.
In the letter Ms Leadsom said she agrees with "Sue Gray's conclusions that there have been significant failures of leadership, both political and official, in No 10 and the Cabinet Office".
She ended the letter by encouraging other Tory backbenchers to "decide individually on what is the right course of action that will restore confidence in our government".
The attack from the former minister is perhaps the most significant for the prime minister so far because despite being moved out the Cabinet in 2020, she's been considered an ally of Mr Johnson from the Brexit-supporting wing of the party.
Ms Leadsom did not call for Mr Johnson to resign and it is unclear whether she has submitted a letter of no confidence in the prime minister - but there is a growing list of backbench Tories who have.
Carlisle MP John Stevenson revealed he'd submitted a letter on Tuesday, saying in a statement that he was "disappointed" by rule-breaking in Downing Street during the pandemic.
He said his "only option" was to try force a vote of no confidence since Mr Johnson was "unwilling" to submit himself to one.
As Boris Johnson's advisor suggests that the fixed penalty notice he received was a breach of the ministerial code, Libby Wierner discusses what it means for the PM.
The backbencher indicated his no confidence letter had been submitted, adding in his statement that he'd "already taken appropriate action".
Chatter amongst Tory MPs, according to ITV News UK Editor Paul Brand, is that Mr Johnson has "begun calling his critics, hinting at the prospect of promotion and asking in return that they 'stay with me'".
Paul said this indicates "No 10 feel the threat of a confidence vote if those kind of approaches are being made".
Number 10 told ITV News it was not aware of those calls being made but said Mr Johnson speaks to his MPs regularly.
Labour Deputy Leader Angela Rayner said the PM "should get off the blower and instead get a grip on the passport delays and travel chaos that is threatening to blight the Jubilee Weekend for so many". “This prime minister has been reduced to desperately phoning around his mutinous MPs offering out baubles in a doomed attempt to save his own skin. “His dishevelled government is asleep at the wheel at the very time that their rank incompetence has left us with the highest inflation in the G7 and a cost of living crisis that is worsening every day." Andrew Bridgen, another former ally of Mr Johnson, submitted a letter of no confidence on Monday afternoon, following Elliot Colburn who did so earlier in the day.
Mr Colburn, the MP for Carshalton and Wallington, is among a group of other Tories from the 2019 intake who have called for the prime minister's resignation.
Again, this is significantly damaging for a PM who only less than three years ago helped them get elected to Parliament for the first time.
Want a quick and expert briefing on the biggest news stories? Listen to our latest podcasts to find out What You Need To know
It appears 2019 MPs Mr Colburn, Aaron Bell, David Simmonds and Nickie Aiken have now decided the best way for them to retain their seat in the next general election would be to have someone else lead them.
More than 20 Tory MPs have now called on Mr Johnson to quit, however it takes 54 of them to demand a vote of no confidence in order for one to be triggered.
There was a flurry of Tories urging Mr Johnson to quit after Ms Gray's report into Partygate condemned widespread rule breaking on Downing Street and blamed a "failure of leadership" in No10 for the party culture there.
Which Tories are publicly calling on Boris Johnson to quit?
All Tories on the below list have publicly called on Mr Johnson to resign but they may not have submitted letters of no confidence, formally demanding a vote on his leadership.
Sir Bob Neil
Anne Marie Morris (who, after having the Tory whip restored, has resubmitted her letter)
It's possible and perhaps likely that some Tories have submitted letters of no confidence but not publicly revealed it.
What is the process for backbench Tories to remove their leader?
Tory MPs are able to force a vote of no confidence in their leader if they won't resign.
To do so requires 15% of the parliamentary party to submit letters of no confidence to the 1922 Committee, which is effectively a HR department for backbenchers.
It would take 54 letters of no confidence to trigger a secret ballot, with a simple majority required for either side to win.
If more than 50% of Tory MPs vote to remove him, he will lose his role of party leader and be barred from competing in the forthcoming leadership election.
If the leader wins over half the votes, then they remain party leader and are given a year's immunity from any further confidence votes.
If a party leader loses a confidence vote then they will be banned from standing in the forthcoming contest and 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.