The Sue Gray report into the partygate scandal may be handed to No10 on Wednesday, The Standard understands.
Its publication is expected to reveal damning new details of the party culture in Downing Street when millions of people across Britain were following lockdown or other Covid rules.
Some Tory MPs have renewed their calls for Boris Johnson to quit over the scandal.
Pictures were obtained by ITV News on Monday evening appearing to show Mr Johnson raising a glass at a leaving do for outgoing director of communications Lee Cain on November 13, 2020.
Bottles of wine and fizz are visible on a nearby table.
His red box is also pictured, which may help his aides to argue that he was at a work event.
The gathering in No10 took place eight days after Mr Johnson imposed England’s second national coronavirus lockdown.
For four weeks, people were banned from social mixing, other than to meet one person outside.
Transport Secretary Grant Shapps defended Mr Johnson.
He told Sky News: : “The question is, was he down there partying? No, clearly not. He had gone by to raise a glass to a colleague who was leaving.
“The answer to this is that the police have spent a lot of time lot of people and a lot of resources crawling over it and come to their conclusion and we know he wasn’t fined for that.”
Asked if Mr Johnson had misled Parliament, he repied: “I think probably what happened is he is at the end of a busy day. He walks down, he says cheers to somebody who has worked there and walks out and to him that is not a party.”
However, some Tory MPs are calling for a confidence vote in Mr Johnson’s leadership.
Waveney MP Peter Aldous said he had not withdrawn his letter of no confidence in Mr Johnson to Sir Graham Brady, chairman of the 1922 Committee of Backbench Tory MPs.
“I haven’t, no. I thought long and hard back in late January, early February, as to whether I should do that,” he told GB News.
“And I weighed up all manner of considerations and I concluded then that it would be best for the country, and dare I say it for the Conservative Party, if he did [resign].
Conservative MP David Simmonds demanded Mr Johnson explain why the pictures did not show a party taking place, with rule-breaking.
The MP for Ruislip, Northwood and Pinner in west London told BBC Radio 4’s Today programme: "I think most colleagues committed to their constituents that they would wait until they had sight of the full report from Sue Gray, I think that is still the case.
"But clearly it does create an issue. We had a vote which the Conservative Party was neutral on in Parliament that there would be an investigation about what was said. Clearly it does raise a new question that we were all told very clearly that there definitely had not been a party on the day in question and these photographs have emerged which suggest that that’s not the case. We need to hear the Prime Minister’s explanation for that."
He said it would be "very difficult" for Mr Johnson to provide a satisfactory answer, but added: "It seems to me he could construct some defence about how people were at work, but we need to see this in context. Many of my constituents lost relatives, they lost friends and family members, my father-in-law died of Covid."
