Boris Johnson must "urgently explain" why he had a "secret" meeting with Sue Gray over her Partygate report, Labour have said.
It follows reports the top civil servant met with the Prime Minister a month ago, to discuss the publication of her long-awaited probe into lockdown-busting Downing Street parties.
Mr Johnson is said to have spoken to Ms Gray about where she thought Scotland Yard were with their own investigation and interviews with civil servants, according to Sky News.
And the BBC reported the pair discussed whether potentially incriminating photographs would be included in the published report.
But it's said the contents of the report itself were not discussed at the meeting.
Her report is set to be published next week, after the Metropolitan Police drew their investigation to a close, handing out 126 fines.
But the Mirror revealed on Thursday that officers only investigated the Prime Minister over two events - despite his reportedly being present for as many as six.
Some 30 people, including the PM, are reportedly set to be contacted over the contents of the report.
Angela Rayner MP, Labour's Deputy Leader, demanded the PM "urgently explain" holding the meeting "despite claiming her investigation was completely independent."
She said: “Public confidence in the process is already depleted, and people deserve to know the truth.
"This is a Prime Minister incapable of taking responsibility for the rotten culture he has created in Downing Street or of doing the decent thing.
"The Sue Gray report must be published in full and with all accompanying evidence."
Downing Street insisted that the Prime Minister had been "clear throughout" that the report should be "completely independent".
It is thought that most of the Cabinet Office letters were sent out on Thursday, as the Metropolitan Police concluded its investigation into lockdown breaches in Downing Street and across Whitehall.
Mr Johnson is set to be among those approached but No 10 had no update as to whether he had received a letter on Friday evening.
Adam Wagner, a human rights lawyer who has specialised in lockdown rule-breaking, said the document was "not an independent report" and "never has been".
"It's an internal report. This isn't a judgment on Sue Gray, it's a statement of fact - she is an employee investigating her direct line managers," he tweeted.
A No 10 spokesperson said: "The Prime Minister commissioned the investigation led by Sue Gray and has been clear throughout that it should be completely independent.
"As he reiterated again today, the decision on what and when to publish rests entirely with the investigation team and he will respond in Parliament once it concludes."
Not everyone contacted by the Cabinet Office will necessarily be named or identified by some other means in the report - it may merely be that their actions are referenced because they are pertinent to the narrative of a particular event.
The investigators are understood not to be including the full details of the document's contents in the letters but instead are giving a "gist" of the relevant sections to those being approached.
Sources close to the inquiry expect the report will be published early next week.
Mr Johnson declined to apologise again for the rule-breaking in No 10 during a visit to Hilltop Honey's factory in Newtown in Powys, Wales, on Friday.
Asked if Downing Street would be blocking any names from appearing in the report, he said: "That will be entirely up to Sue Gray and I'll be looking forward very much to seeing what she has to say, and fingers crossed that will be pretty soon next week."
There were calls for Scotland Yard to explain how Mr Johnson received just the one fine, for his 56th birthday gathering in June 2020 when indoor mixing was banned.
Former director of public prosecutions Lord Ken Macdonald warned "it's very difficult for us to disentangle exactly how the police investigation has proceeded and how fair it's been".
He told BBC Radio 4's Today programme: "I think without the police providing an explanation for that it's very difficult for us to understand why they came to the conclusions that they did."
The crossbench peer said it was not known why the Prime Minister was fined for his presence at what was deemed one of the less serious events but not others.
There has also been no explanation as to why Cabinet Secretary Simon Case, who had to recuse himself from running the civil service inquiry after reports of a Cabinet Office Christmas party surfaced, has not been fined.
The Met declined to identify anyone in its £460,000 investigation.
A team of 12 detectives examined 345 documents, including emails, door logs, diary entries, witness statements and 204 questionnaires.
They also examined 510 photographs and CCTV images.
Mr Johnson, his wife Carrie Johnson and Chancellor Rishi Sunak were all fined over the birthday gathering.
But both Johnsons were told by police they face no further action, according to Downing Street, and Mr Sunak has not received an additional fine.
The Met said 28 individuals had received between two and five fines.
