Boris Johnson faces a "drip, drip" of disillusioned Tory MPs calling for him to quit until Scotland Yard has completed its 'Partygate' probe.
Three more Tories have submitted letters of no confidence in the Prime Minister over the row in just one day.
It means the number of letters required to trigger a no confidence vote is creeping up towards the 54 threshold.
One Tory minister told the Mirror: "There will be a slow drip, drip of letters over the coming weeks until the Met finishes it inquiry. It's going to be excruciating."
Scotland Yard insiders claim that dozens of detectives have been temporarily transferred to bolster the investigation.
A source said: "There's about 30 detectives working on this which is a lot considering they're dealing with fixed penalty notices. It could be reasonably quick".
Detectives have around 300 photographs and 500 bits of paper to consider and have said they want to complete their inquiry "promptly".
They may also speak to key figures at the heart of the affair including Mr Johnson and his wife Carrie.
Former Cabinet Secretary Sir Mark Sedwill who was one of several senior officials at a gathering on June 18, 2020, is also expected to be interviewed.
The PM is facing anger from his own party after senior civil servant Sue Gray's interim report blasted "failures of leadership" over No 10 parties.
Mr Johnson is now directly linked to six of the 12 gatherings that the Met Police are investigating.
These include a party in the Downing Street flat the PM shares with wife Carrie on the night Dominic Cummings left No 10.
Former Cabinet ministers David Davis and Andrew Mitchell are among more than a dozen MPs who have publicly said the PM should go.
But cabinet minister Nadine Dorries, a key member of Team Boris, attacked Tory MPs who have submitted letters as "a handful of egos".
She added: "It's selfish, doing Labour's work and it's not really helping their constituents".
Cabinet minister Michael Gove ruled out running for Tory leader again when asked in the Commons.
"Been there, knocked back twice, won't be doing that again," he said.
First out the blocks to reveal he was handing in a letter on Wednesday was former minister Tobias Ellwood.
The chair of the defence select committee made the bombshell announcement just as the PM jetted back from Ukraine to face the music from his own party at PMQs.
Mr Ellwood told Sky News: “This is just horrible for all MPs to continuously have to defend this to the British public".
He added: “I don’t think the Prime Minister realises how worried colleagues are in every corner of the party, backbenchers and ministers alike.
"This is all only going one way and will invariably slide towards a very ugly place.
"I believe it’s time for the Prime Minister to take a grip of this; he himself should call a vote of confidence rather than waiting for the inevitable 54 letters to be eventually submitted.
"It’s time to resolve this completely so the party can get back to governing."
Tory backbencher Anthony Mangnall also broke ranks, saying Mr Johnson's actions were "overshadowing” good work elsewhere.
The MP for Totnes and South Devon tweeted: “Standards in public life matter. At this time I can no longer support the PM.
“His actions and mistruths are overshadowing the extraordinary work of so many excellent ministers and colleagues."
A third Tory MP, Gary Streeter, followed swiftly afterwards, announcing he had submitted a letter.
The MP for South West Devon said: "I cannot reconcile the pain and sacrifice of the vast majority of the British public during lockdown with the attitude and activities of those working in Downing Street.
"Accordingly, I have now submitted a letter seeking a motion of no confidence in the Prime Minister. I have not come to this decision lightly."
The number is expected to swell further if the PM fails to withdraw his claims about Mr Starmer failing to prosecute notorious paedophile Jimmy Savile.
The PM repeated his claim at PMQs after the Labour leader accused him of “parroting the conspiracy theories of violent fascists to score cheap political points”.
He added: “He knows exactly what he is doing. It’s time to restore some dignity."
In a jibe over the Met's partygate investigation, Mr Starmer said: “That’s lots of words, lots of bluster, no answers.
"A word of warning, Prime Minister, that’s not going to work with the police”.
Tory MPs in the 40-strong One Nation group have accused Mr Johnson of acting like a “pound shop Donald Trump ” and suggested that more letters would go in.
One told The Times: “The basic decencies are something we want to see in politics. There is a widespread feeling that this broke any kind of convention."
Mr Starmer led the Crown Prosecution Service when it decided not to prosecute Savile in 2009 due to insufficient evidence, a decision it later apologised for.
But the Full Fact website investigated in 2020 and found it had never actually been suggested that Sir Keir was personally involved in the decision.
