Boris Johnson will have no choice but to resign as prime minister when the contents of Sue Gray's report are made public, a source has claimed.
Johnson is under growing pressure after being issued with a Fixed Penalty Notice (FPN) for attending a party in the Cabinet Office in June 2020.
The Metropolitan Police has so far issued at least 50 fines relating to lockdown-breaking parties within Downing Street and Whitehall.
With local elections looming on 5 May, the force has said it will not give any further updates on the investigation until after voters have gone to the polls.
Prior to the Met Police's criminal investigation, senior civil servant Sue Gray was tasked with investigating allegations of rule-breaking in Whitehall - and the contents reportedly make difficult reading for the PM.
As part of the probe, she was handed more than 300 images and 500 pages of information in relation to the allegations.
A slimmed-down version of the report was released in January, which found there was a "serious failure to observe high standards" of those in Downing Street.
But the full publication of the report was delayed following the Met's announcement it was conducting its own criminal investigation into the gatherings.
Watch: Tory chair deflects question on comparing Partygate fines to parking fines
A source with knowledge of the report told The Times: "Sue's report is excoriating. It will make things incredibly difficult for the Prime Minister.
"There's an immense amount of pressure on her - her report could be enough to end him."
Speaking following the reports, Johnson said there was “absolutely no circumstance” in which he was going to comment on the final report ahead of its publication.
“There is absolutely no circumstance in which I’m going to comment on that before the thing is complete,” he said in a pooled clip for broadcasters during a visit to Bury.
Downing Street has so far refused to comment on the claims, but said Johnson has not received any further FPNs for breaches of lockdown regulations.
The prime minister’s official spokesman said: “There is nothing to update on."
Johnson has already paid his fine for attending a gathering in the Cabinet Office in June 2020 to celebrate his birthday.
The fine made him the first prime minister in history to have been found to have broken the law while in office.
Downing Street is bracing for the PM to be issued with a further fine in connection with a party in the gardens of Number 10, in which staff were encouraged to "bring your own booze" to "make the most of the lovely weather".
Johnson is said to have been at six of the 12 events which the Met Police are investigating.
He has previously said the Sue Gray report would be published in full.
What was in Sue Gray's report?
Gray's 12-page report found there was a "serious failure to observe high standards" expected of those working at the heart of British democracy.
In total Gray looked into 16 events, 12 of which the Met Police have investigated.
Of those, the prime minister is directly implicated in at least three – his birthday party gathering in 19 June 2020 which he has now been fined for; the Downing St garden party he attended on 20 May 2020; and a gathering in his private flat on 13 November.
The latter of these could prove problematic to the prime minister, who has previously flatly denied that any such gathering took place.
The report also found there "were failures of leadership and judgment" and the gatherings represented "a serious failure to observe not just the high standards expected of those working at the heart of Government but also of the standards expected of the entire British population at the time".
Johnson is facing a souring mood in his own party, and last week failed to whip Tory MPs who backed a Commons inquiry into whether he had misled Parliament.
Rumours of an impending vote of no confidence have been swirling round Westminster, with sources claiming 46 letters of no confidence have been sent to Sir Graham Brady, chairman of the backbench Conservative 1922 Committee - just eight short of what would be needed to trigger a vote.