Boris Johnson's birthday celebration is one of 12 gatherings being investigated by the Metropolitan Police.
A limited version of Sue Gray's report into the alleged parties at No.10 and Whitehall was published today.
The findings criticised a "serious failure" to observe the high standards expected of those working in the heart of government during the height of coronavirus restrictions.
READ MORE: Met Police demand Sue Gray's report has 'minimal reference' to Downing Street parties
The top civil servant confirmed the force is looking at an alleged gathering on June 19 2020 in the Cabinet Room when the Prime Minister was surprised with a birthday cake.
ITV news reported up to 30 Downing Street staff "gathered briefly" in the Cabinet Room after a meeting.
It is claimed the party was organised by the Prime Minister's wife, Carrie Johnson, and staff joined in a chorus of "happy birthday" at a time when indoor social mixing was banned.
A Number 10 spokeswoman said: "A group of staff working in No 10 that day gathered briefly in the Cabinet Room after a meeting to wish the Prime Minister a happy birthday. He was there for less than 10 minutes."
The party is one of 12 being investigated by police, which also includes a gathering in the Downing Street flat.
In a carefully-worded statement, the Cabinet Office said Ms Gray had provided an "update" on her investigation to the Prime Minister, suggesting it was not the full report she was preparing before the Metropolitan Police intervened.
Scotland Yard last week asked Ms Gray to make only "minimal reference" to gatherings being investigated by its officers.
Downing Street has so far refused to commit to publishing a fuller version of Ms Gray's findings once the police investigation has concluded.
Ms Gray's report listed 16 events she examined as part of her inquiry, but she said only four of those were not now being investigated by the police
However, the civil servant decided not to publish factual accounts relating to events not under investigation by police because it would damage her overall findings.
She said: "In respect of the gatherings that the Metropolitan Police has assessed as not reaching the threshold for criminal investigation, they have not requested any limitations be placed on the description of those events, however, I have decided not to publish factual accounts in relation to those four dates.
"I do not feel that I am able to do so without detriment to the overall balance of the findings."
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