Boris Johnson is facing calls to explain the purpose of a "secret" meeting with Sue Gray as the long-awaited partygate report is set to be published this week.
Reports have suggested the document, expected to be published in the coming days, will feature photographs of illegal gatherings.
It was also reported that top civil servant Simon Case will be particularly hard-hit by the contents, despite the fact he was not fined over the scandal.
It follows the conclusion of a separate inquiry by the Metropolitan Police into Covid rule-breaking events at the heart of Government, which saw a total of 83 people receive at least one fixed-penalty notice (FPN) each for attending events over eight separate days.
Boris Johnson received just one fine, for his 56th birthday gathering in June 2020 when indoor mixing was banned, along with his wife Carrie Johnson and Chancellor Rishi Sunak.
Both Johnsons were later told by police they faced no further action, according to Downing Street, and Sunak has not received an additional FPN.
The Telegraph cited a source as saying Case will come in for "stinging criticism" in the Gray report.
"As the head of the civil service, the ultimate responsibility was his," they added.
The Cabinet Office declined to comment on the reports about the permanent secretary.
Meanwhile, Tory MP Laura Farris suggested she may resign as a ministerial aide at the Foreign Office in order to continue in her role on the Commons Privileges Committee, which is set to investigate whether the PM intentionally misled Parliament over partygate.
Ms Farris, who is currently both a parliamentary private secretary (PPS) and a member of the committee, told BBC Radio 4's Westminster Hour programme the two roles were "incompatible" in the circumstances, and "that has to be resolved this week".
"One or other will go. If I am to remain on the committee, I will resign as a PPS so that there isn't that conflict," she said.
Asked which option she was leaning towards, she said she thought it would be "the right thing to do" to stay on the committee.
The committee's chairman, Labour MP Chris Bryant, previously recused himself from the parliamentary investigation, having made his views on Johnson's conduct plain in the media.
Cabinet ministers failed to shed light on the circumstances of the controversial meeting between Mr Johnson and Ms Gray, details of which first emerged on Friday, as Labour said people "deserve to know the truth".
Education Secretary Nadhim Zahawi insisted during a round of broadcast interviews on Sunday that he did not know who called the meeting, or what was discussed, while maintaining Gray had "complete control" over what would be published in the document.
International Trade Secretary Anne-Marie Trevelyan also said she did not know who had organised the talks, adding: "I don't follow anybody's diaries."
It is understood Johnson and Gray met at least once for an update on the report's progress while it was being drafted, but a Whitehall source said its contents were not discussed at any point.
The exact nature of the talks remains unclear.
Such meetings would not have been viewed as unusual, the source said, with the aim to take stock of what stage the report was at.
It was initially reported by the BBC that the discussions touched on whether photos would be publicly disclosed, and that Gray initiated the meeting "to clarify her intentions" over what would happen once the police investigation concluded.
However, a spokesman for the Gray inquiry disputed this account of events.
A No 10 source insisted the request for the meeting did not come from Johnson.
It has been reported the idea was in fact suggested by a No 10 official, while the calendar invitation was sent by Gray.
This version of events was corroborated to the PA news agency by one source close to the inquiry, but there were conflicting accounts over the circumstances that led to the talks.
It came as about 30 people, including Johnson, were being contacted by the Cabinet Office to warn them of the contents of the document.
It is thought that most of the letters were sent out on Thursday, as the Met concluded its investigation.
Angela Rayner, Labour's deputy leader, called on the Prime Minister to "urgently explain" why the "secret meeting" with Gray took place.
Downing Street insisted Johnson had been "clear throughout" that the report should be "completely independent".
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