The report on which the Prime Minister’s future may depend is expected to be delivered later this week.
But who is Sue Gray, the senior official leading the probe, and what might her investigation uncover?
Here is a look at what the hotly-anticipated inquiry is all about.
What is the inquiry into?
A catalogue of allegations have been made about rule-breaking parties held in No 10 and elsewhere in Government while tough coronavirus restrictions were in place during 2020 and 2021, ranging from summer garden drinks to Christmas bashes.
Its remit will include understanding what happened on May 20 2020 during a “bring your own booze” garden drinks event for staff held when it was forbidden for more than two people to meet outside during the first coronavirus lockdown, and that the Prime Minister has admitted he attended for about 25 minutes.
The probe has also been expanded as other claims have come to light, and will also take in two staff leaving dos on April 16 last year on the eve of the funeral of the Duke of Edinburgh, which have already seen No 10 apologise to Buckingham Palace.
Why is Sue Gray leading it?
The senior official found herself thrust into the limelight and chosen to step in to lead the investigation after Cabinet Secretary Simon Case, her boss, recused himself following allegations that his own office held a Christmas event in December 2020.
Has she got form for such inquiries?
Yes, in that Ms Gray, a former publican, has investigated two Cabinet ministers in the past over alleged wrongdoing.
The subjects of those investigations, former first secretary of state Damian Green and former chief whip Andrew Mitchell, ended up being sacked and quitting respectively.
Will the report make any judgments?
According to the Institute for Government (IoG), Ms Gray’s final document is “set to be a largely factual account about parties that were held in Downing Street”.
Catherine Haddon, a senior fellow at the think tank, said the report “may not assign individual blame but might refer disciplinary action to others”.
Should Boris Johnson be worried?
The Prime Minister has put a lot of stock in the investigation and will be anxiously awaiting its findings but it is suggested it is unlikely to pass judgment on his actions.
The IoG states that, while Ms Gray’s findings may “touch on the role of the Prime Minister”, it is not her place to “judge his behaviour”.
But Dr Haddon goes on to say that the “bare facts alone could prove deeply damaging”, including in the way the official sets them out and the language used.
It will then be for Conservative MPs and possibly Mr Johnson’s independent ethics adviser, should the Prime Minister commission a separate probe by Lord Geidt, to decide whether he broke the Ministerial Code and misled Parliament.
Mr Johnson has previously said that he has followed coronavirus guidance and, while apologising for not stopping the May 20 gathering, has told the Commons he understood it to be a “work event”.
Which key players will be interviewed?
The Prime Minister has been interviewed by Ms Gray, and is understood to have shared what he knows.
On Monday, the PM’s former chief aide Dominic Cummings is expected to give evidence to the inquiry, after he made bombshell allegations over the parties.
Ms Gray is also reported to have spoken to Metropolitan Police officers stationed at Downing Street, who the Telegraph said were “only too willing” to cooperate.
There are also questions over whether Mr Johnson’s wife, Carrie, will be asked to speak to Ms Gray, after The Sunday Times reported Ms Gray had further widened her inquiry following allegations about gatherings held in the flat the couple share at 11 Downing Street.
Will the Prime Minister accept the findings?
Downing Street insists he will.
The Prime Minister’s official spokesman previously told reporters: “Yes, I think, without pre-empting the findings of the review, it has his backing and he will accept what facts she establishes.”
However, it is unclear what action the PM would take following the report.
When will the conclusions be published?
No 10 has said it wants the process finalised “as soon as possible” but that timings are not in its hands.
Reports suggested the final document was due early this week, but that this is now expected to be a few days later.
Could it be delayed?
If the police decide to investigate any allegations they consider to be a criminal offence, then Ms Gray’s own review could be paused.
The Metropolitan Police have confirmed they are in contact with Cabinet Office officials about the inquiry.
Is the investigation independent?
Given Ms Gray, a former director general of propriety and ethics, has worked in the Cabinet Office on-and-off for more than 20 years and is investigating fellow civil servants and ministers, it is fair to say it is not strictly independent.
Asked how the inquiry was deemed independent last week, Downing Street was unable to point to any safeguards or processes which made it so.
Who will the final report go to?
It will go to the Prime Minister and No 10 has committed to publishing it, with Mr Johnson also pledging to make a statement in the Commons.
However, deputy Prime Minister Dominic Raab refused on Sunday to be drawn of whether the findings would be published in full, although he insisted there would be “full transparency”.
“The process for it will be for the Prime Minister to decide,” he said.
What are the allegations Ms Gray is likely to establish the facts about?
Below is a list of events that have either been alleged or admitted to by the Government.
The terms of reference state that Ms Gray can choose to widen her investigation should further “credible allegations” surface.
May 15 2020: Downing Street wine and cheese gathering in the garden
May 20: “bring your own booze” garden party in No 10
November 13: a leaving party for one of the PM’s senior aides
November 13: a party in Mr and Mrs Johnson’s flat
November 25: Treasury drinks
November 27: A second No 10 staff leaving do
December 10: Department for Education party
December 14: Conservative HQ party featuring Tory London mayoral candidate Shaun Bailey and staff
December 15: Downing Street “virtual” quiz
December 16: Department for Transport party
December 17: Cabinet Office Christmas event
December 18: Christmas party at Downing Street
Run up to Christmas: Leaving do at No 10 for defence adviser Captain Steve Higham
April 16, 2021: Leaving parties for former Downing Street director of communications James Slack and one of the PM’s personal photographers, the day before the Duke of Edinburgh’s funeral.