The 12-page document published on Monday reveals that Ms Gray investigated 16 events over a 20-month period from 15 May 2020 to 16 April 2021.

She interviewed "70 individuals...some more than once" and examined emails, WhatsApp messages, photographs, and building entry logs, as well as searching official records.

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The senior civil servant concluded that "some of the behaviour surrounding these gatherings is difficult to justify", but it is not her place to preside over whether coronavirus laws were broken or not.

Instead the Met Police's probe, launched last week, will look at whether laws were broken at 12 of the 16 gatherings.

The remaining four "are not considered to have reached the threshold for criminal investigation".

Key findings from Sue Gray's report

Due to the Met's request for "minimal reference" to be made to the 12 events detectives are investigating, Ms Gray says it is "not possible at present to provide a meaningful report setting out and analysing the extensive factual information".

This may be published once the Met inquiry has concluded, however.

Here Sky News looks at which gatherings Sue Gray investigated, the ones the police are probing and what information is yet to be made public.

According to the report, the following events were covered by Ms Gray:

15 May 2020: "A photograph showing a number of groups in the garden of No 10 Downing Street"

Police investigating? No

The photo, published by The Guardian, showed Boris Johnson, his wife Carrie, principal private secretary Martin Reynolds and then-special adviser Dominic Cummings in the Downing Street garden.

Former health secretary Matt Hancock was also reported to be there, alongside around 20 other staff.

Sources claim they drank wine, ate food and the prime minister was there for 15 minutes when he said his colleagues deserved a drink for "beating back coronavirus".

20 May 2020: "A gathering in the garden of No 10 Downing Street for No 10 staff"

Police investigating: Yes

Reports of the "bring your own booze" party in the Downing Street garden on 20 May 2020 - just two months into the first lockdown - caused outrage.

First published by ITV News, the report revealed that the prime minister's principle private secretary Martin Reynolds emailed around 100 staff inviting them to "some socially distanced drinks in the No 10 garden".

It read: "Hi all, after what has been an incredibly busy period we thought it would be nice to make the most of the lovely weather and have some socially distanced drinks in the No 10 garden this evening. Please join us from 6pm and bring your own booze!"

Just an hour before, Culture Secretary Oliver Dowden had appeared at a Downing Street news briefing and told people they could only meet one other person from outside their household.

Around 40 staff are thought to have attended, with the PM present for around 25 minutes, he said to "thank staff" for their work on the pandemic.

18 June 2020: "A gathering in the Cabinet Office, 70 Whitehall on the departure of a No 10 private secretary"

Police investigating: Yes

No details have been released about a gathering at the Cabinet Office on 18 June 2020 - in either the media or in Ms Gray's report.

The Met have decided it meets the threshold to form part of their investigation, so details may be released when the force publishes its findings.

19 June 2020: "A gathering in the Cabinet room in No 10 Downing Street on the prime minister's birthday"

Police investigating: Yes

On the afternoon of Boris Johnson's birthday in 2020, ITV News reported that several Number 10 staff gathered in the Cabinet room.

His wife Carrie Johnson allegedly presented him with a cake and everyone sung him Happy Birthday.

Later that day, ITV News claimed a number of the couple's family friends were hosted in the Downing Street flat.

Number 10 has repeatedly insisted the reports are "totally untrue" and that instead a maximum of six guests spent time in the garden that evening, in line with the rules at the time.

13 November 2020: "A gathering in the No 10 flat"

Police investigating: Yes

Mr Johnson's former special adviser Dominic Cummings claimed that on his last day at Downing Street - during the second national lockdown in England - a party took place inside the prime minister's private flat.

Mr Cummings was pictured that evening leaving Number 10 with a box of his belongings and claims it happened after he left.

"A gathering in No 10 Downing Street on the departure of a special adviser"

Police investigating: Yes

The Times also claimed that on the same evening a leaving party was held for the PM's outgoing head of communications Lee Cain.

Mr Johnson was reported to have made a speech and Mr Cain was pictured wearing boxing gloves.

27 November 2020: "A gathering in No 10 Downing Street on the departure of a special adviser"

Police investigating: No

During the second national lockdown, a leaving party was reported to have taken place for Cleo Watson, a former aide of Dominic Cummings.

The prime minister is reported to have made a speech at the gathering.

10 December 2020: "A gathering in the Department for Education ahead of the Christmas break"

Police investigating: No

The Departure for Education has confirmed that a Christmas party was held inside their building on 10 December when London was in Tier 2 COVID restrictions.

It was attended by-then Education Secretary Gavin Williamson, the Daily Mirror reported at the time.

15 December 2020: "A gathering in No 10 Downing Street for an online Christmas quiz"

Police investigating: No

The Sunday Mirror published a photograph of Boris Johnson taking part in an online Christmas quiz.

It showed him flanked by colleagues, while other staff in various groups were huddled together across the Downing Street complex.

The newspaper claimed alcohol was consumed during the quiz, which the PM hosted for around 10 or 15 minutes.

17 December 2020: "A gathering in Cabinet Office, 70 Whitehall to hold an online Christmas quiz for the Cabinet Secretary's private office"

Police investigating: Yes

Cabinet Secretary Simon Case had to give the role of carrying out investigations into Downing Street parties to Sue Gray after it emerged one had taken place in his office.

The so-called "Christmas party" event involved six people working in the Cabinet Office and a further six working from home.

Although Mr Case did not take part, Sky News understands he walked through the room to get to his own private office.

Staff were reported to have enjoyed drinks and snacks throughout.

"A gathering in Cabinet Office, 70 Whitehall on the departure of a senior Cabinet Office official"

Police investigating: Yes

The same evening, leaving drinks took place in the Cabinet Office for the outgoing director-general of the COVID taskforce Kate Josephs.

Confirming the event took place and offering her apology, Ms Josephs wrote on Twitter: "On the evening of 17 December, I gathered with colleagues that were at work that day, with drinks, in our office in the Cabinet Office, to mark my leaving the civil service.

"I am truly sorry I did this and for the anger that people will feel as a result."

"A gathering in No 10 Downing Street on the departure of a No 10 official"

Police investigating: Yes

It was reported that leaving drinks took place at some point in December 2020 for the prime minister's defence aide Captain Steve Higham.

The date of the event was never confirmed and the Ministry of Defence declined to comment, but it could have taken place on 17 December.

Sky News understands Mr Johnson was "there for a few minutes to thank him for his service".

18 December 2020: "A gathering in No 10 Downing Street ahead of the Christmas break"

Police investigating: Yes

A Christmas cheese and wine evening is believed to have taken place inside Number 10 Downing Street on the evening of 18 December when London was in Tier 3 COVID restrictions.

Around 40 staff are thought to have been present, with Secret Santa gifts exchanged and awards handed out by Downing Street's director of communications and former Daily Mail journalist Jack Doyle.

Sky News understands people stayed until 2am.

It was this event that was joked about during a "fake press conference" given by then-adviser to Boris Johnson, Allegra Stratton. She resigned soon afterwards.

14 January 2021: "A gathering in No 10 Downing Street on the departure of two No 10 private secretaries"

Police investigating: Yes

There have been no details released of the events Sue Gray refers to on 14 January in the media or in her report.

Some may be published in the Met Police report as the Gray report says it met the threshold for criminal investigation and forms part of their probe.

16 April 2021: "A gathering in No 10 Downing Street on the departure of No 10 official"

Police investigating: Yes

The Daily Telegraph first reported that two gatherings were held inside Number 10 the evening before the Queen was forced to sit alone at her husband Prince Philip's funeral.

They took place in separate parts of Downing Street but later came together - when COVID restrictions banned indoor mixing.

The first was a leaving party for the PM's then-head of communications James Slack - now deputy editor-in-chief at The Sun.

There were around 30 guests in total at both parties, with one involving a member of staff being sent to The Co-op on The Strand with a suitcase to fill with alcohol.

Reports claimed someone broke the PM's son's swing in the Downing Street garden, while one aide DJed for other staff.

"A gathering in No 10 Downing Street on the departure of another No 10 official"

Police investigating: Yes

The other event in Downing Street that evening was a leaving party in an honour of one of the prime minister's official photographers.

Both events are believed to have lasted through to the early hours.

Boris Johnson was not at either event as he was staying at Chequers.