The chair of the Tory 1922 committee is expected to announce a vote of no confidence in Boris Johnson as soon as today, according to reports.
Tory rebels expect Sir Graham Brady to make the announcement about the Prime Minister's future this morning, ITV reports.
Sir Graham was expected to make a short statement to cameras today as a Jubilee reprieve granted to the PM shattered.
Ex-Treasury minister Jesse Norman published a brutal letter on social media, calling on the PM to go.
He said Mr Johnson had presided over "a culture of casual law-breaking" in No 10 and that his claim to be vindicated by the Sue Gray report was "grotesque".
Under Conservative Party rules, if enough MPs write to the backbench chief then a secret ballot is held to express "no confidence" in the leader.
Tory rebels believe 54 letters - the number needed to trigger a vote - has already been reached and will be announced on Monday morning.
Mr Johnson's leadership has taken a hit over the Partygate scandal in which he was only issued one penalty by Scotland Yard over illegal gatherings during the coronavirus lockdown.
Mr Johnson said he was "humbled" by the findings but tried to shift blame onto staff by claiming he wasn't aware of how raucous some events became.
It initially appeared that Tories were buying his defence but a string of MPs have now broken cover to call for the PM to resign.
The mood darkened ahead of the Jubilee weekend, with widespread speculation in Westminster that rebels had the numbers to trigger a vote in Mr Johnson's leadership.
Only 1922 chief Sir Graham Brady knows whether enough Conservatives have written letters of no confidence in Mr Johnson to trigger a vote.
54 MPs - or 15% of the Tory parliamentary party - must write to Sir Graham to fire the starting gun on a leadership vote.
Health Secretary Sajid Javid admitted there "may well" be a vote - but insisted the PM would stay and fight.
"This country doesn't need a vote of confidence in the Prime Minister. What we need to be doing as a country is looking ahead to the challenges that exist," Mr Javid told Sky News.
"If this threshold of 54 letters is reached there will be a confidence vote and in that case there should be. There may well be one.
"If there is, the Prime Minister will stand and fight his corner with a very, very strong case. So let's just wait and see what happens."
