Police are investigating 12 parties in Downing Street and Whitehall, including a birthday gathering for Boris Johnson, according to Sue Gray's report.
The top civil servant revealed she was looking at 16 events in her partygate probe - and 12 of those are being examined by the Metropolitan Police.
Only four events do not meet Scotland Yard's threshold for investigation, including a gathering in the Downing Street garden on May 15 2020 where the PM and staff were pictured enjoying cheese and wine.
READ MORE: Sue Gray finds 'failures of leadership' in Downing Street investigation
A leaving do on November 27 2020 for special adviser Cleo Watson, a Christmas drinks at the Department for Education on 10 December 2020 and a No10 Christmas quiz on 15 December 2020 will also be left out of the police inquiry.
But Ms Gray confirmed that the police are examining a birthday gathering on June 19 2020 in the Cabinet room, where the PM was surprised by a cake.
His wife Carrie Johnson is said to have led staff in a chorus of "Happy Birthday".
Ms Gray said it was "not possible at present to provide a meaningful report" setting out all she discovered, due to the ongoing police investigation.
In her general report findings, Ms Gray said: "Against the backdrop of the pandemic, when the Government was asking citizens to accept far-reaching restrictions on their lives, some of the behaviour surrounding these gatherings is difficult to justify.
"At least some of the gatherings in question represent a serious failure to observe not just the high standards expected of those working at the heart of government but also of the standards expected of the entire British population at the time.
"At times it seems there was too little thought given to what was happening across the country in considering the appropriateness of some of these gatherings, the risks they presented to public health and how they might appear to the public."
She said there were 'failures of leadership and judgment by different parts of No 10 and the Cabinet Office at different times' and added that "Some of the events should not have been allowed to take place, while others should not have been allowed to develop as they did."
Ms Gray also criticised the 'excessive consumption of alcohol' in Downing Street as 'not appropriate in a professional workplace at any time' - calling for a robust new policy on the matter.
She made specific reference to reports of gatherings in the Downing Street garden, adding: "The use of the garden at No 10 Downing Street should be primarily for the Prime Minister and the private residents of No 10 and No 11 Downing Street.
"During the pandemic it was often used as an extension of the workplace as a more covid secure means of holding group meetings in a ventilated space.
"This was a sensible measure that staff appreciated, but the garden was also used for gatherings without clear authorisation or oversight. This was not appropriate. Any official access to the space, including for meetings, should be by invitation only and in a controlled environment."
Ms Gray said that her investigations found that some Number 10 staff wanted to raise concerns about behaviours they witnessed at work, but at times felt unable to do so.
She added: "There should be easier ways for staff to raise such concerns informally, outside of the line management chain."
In the conclusion of her short 12-page report, Ms Gray wrote: "The gatherings within the scope of this investigation are spread over a 20-month period – a period that has been unique in recent times in terms of the complexity and breadth of the demands on public servants and indeed the general public.
"The whole of the country rose to the challenge. Ministers, special advisers and the Civil Service, of which I am proud to be a part, were a key and dedicated part of that national effort. However, as I have noted, a number of these gatherings should not have been allowed to take place or to develop in the way that they did.
"There is significant learning to be drawn from these events which must be addressed immediately across Government. This does not need to wait for the police investigations to be concluded."
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