The Prime Minister is among 30 people who will be contacted over the contents of the Sue Gray Partygate report.
It is thought most of the letters were sent on Thursday - the same day the Metropolitan Police concluded its Partygate investigation after more than 100 fines.
Boris Johnson is set to be among them, but as of Friday evening, there was no update on whether he had received the letter.
It is understood Ms Gray and the Prime Minister had at least one meeting regarding the report's progress while it was drafted - but the contents were not discussed.
It is important to note that not everyone contacted by the Cabinet Office will necessarily be named or identified by some other means in the report.
It could mean that their actions are referenced because they are pertinent to the narrative of a particular event.
It is understood that investigators will not be including the full details of the report in the letter but only covering a "gist" of the contents.
It comes as Mr Johnson insisted he would not seek to block names appearing in the senior civil servant’s report, and said he was “looking forward” to seeing its contents “pretty soon”.
The Prime Minister said he was “very grateful” for the Met’s investigation, which concluded with 126 fines.
It is believed the report will be published early next week.
On reports the PM had met with Ms Gray during the drafting process, 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.”
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 Met declined to identify anyone in its £460,000 investigation.
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.
