The results of Sue Gray's Partygate report into lockdown breaches could be enough "to end Boris Johnson" a source has claimed.
The Prime Minister has apologised after being fined over a birthday party which also saw Chancellor Rishi Sunak punished.
Mr Johnson has since pleaded for the nation to move on as he conducts trade talks in India.
But the PM is expected to be hit by more allegations by Sue Gray, with an inside source fearing his position will be untenable by the end of the enquiry.
Boris Johnson in India
Stefan Rousseau
A source told the Times newspaper: "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."
Prime Minister Boris Johnson leaves 10 Downing Street, London, to attend Prime Minister's Questions at the Houses of Parliament
Jonathan Brady
Downing Street is yet comment on these claims.
These claims come after reports rebel Tory MPs are ready to submit letters of no confidence in the Prime Minister after the local elections.
Former chief whip and Brexiteer Mark Harper called for Mr Johnson to step down over Partygate live in the Commons.
He said: "I regret to say that we have a Prime Minister who broke the laws that he told the country they had to follow, hasn’t been straightforward about it.
"And [he] is now going to ask the decent men and women on these benches to defend what I think is indefensible.
"’I'm very sorry to have to say this, but I no longer think he is worthy of the great office that he holds.”