The findings of Sue Gray's inquiry into alleged coronavirus rule-breaking across Downing Street and Whitehall are expected to finally be released this week.
The senior civil servant is looking into a series of allegations suggesting that the prime minister and his aides attended parties in breach of regulations put in place to stop the spread of the virus throughout the pandemic.
Mr Johnson has admitted to attending a Downing Street garden party, which he insists he thought was a work event, and a surprise birthday party put on for him in the Cabinet Office, which he only attended "for less than 10 minutes", according to No 10.
But he has so far refused to be drawn on the details of the allegations, insisting that the findings of Ms Gray's report should not be pre-empted.
On Tuesday, he welcomed the news that the Met Police had launched an investigation into potential breaches of coronavirus laws at a “number of events” in Downing Street and Whitehall, saying that he hopes the investigations help to 'draw a line under the matter'.
However, the criminal investigation threatened to derail the publication of Ms Gray's report as No 10 suggested the inquiry would be unable to look into events that were part of the police probe.
It was suggested that the full report may be delayed, or only published in part while the police investigation was carried out.
It is now understood that Scotland Yard has given the green light for publication, which could not come as soon as Wednesday.
Here, we look at what the Sue Gray inquiry involves and when her report could be released to the public.
Who is Sue Gray?
Senior civil servant Sue Gray found herself thrust into the limelight when she was chosen to step in to lead the investigation.
Ms Gray, a former publican, is the second permanent secretary of the Cabinet Office.
Initially, Simon Case – her boss – was leading the inquiry, but he recused himself following allegations that his own office held a Christmas event in December 2020.
Ms Gray 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.
What is Sue Gray's inquiry?
The inquiry is thought to be looking into a number of alleged events, but details of which ones have not yet been disclosed.
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”.
The alleged events confirmed to be included in the inquiry's remit include a May 20 2020 “bring your own booze” garden drinks event for staff attended by the PM and two staff leaving dos on April 16 last year on the eve of the funeral of the Duke of Edinburgh.
Is Sue Gray impartial?
Ms Gray's report is supposed to be an independent investigation and, as a civil servant, she is expected to be impartial.
However, the government has been questioned over the transparency of the inquiry.
Given Ms Gray has worked in the Cabinet Office on-and-off for more than 20 years and is investigating her fellow civil servants and ministers, so it may be fair to suggest the inquiry 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.
Mr Johnson's former aide Dominic Cummings has also suggested that staff participating in the inquiry may have kept back key evidence because it would be seen by the PM.
Downing Street has suggested that it may ultimately be up to Boris Johnson to decide what is published from the report.
The Prime Minister’s official spokesman said “it’s very much our intention to publish the findings in full as set out in the terms of reference”.
But asked if it was up to the Prime Minister or Ms Gray what could be published, the spokesman said: “I think it is a report that comes to the Prime Minister.”
Mr Johnson had previously said the report would be published in the House of Commons library.
Labour’s deputy leader Angela Rayner has said the report “must be published in its entirety with all accompanying evidence”.
Who has been interviewed for the inquiry?
It is understood that Mr Johnson has been interviewed by Ms Gray.
The PM’s former chief aide Dominic Cummings was also 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, has been 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.
When will Sue Gray's report be released?
The final report will go to the prime minister and No 10 has committed to publishing it.
Mr Johnson has also pledged to make a statement in the Commons.
According to the latest reports, Ms Gray's full report into the alleged parties held at Downing Street during lockdown could be published as soon as tomorrow.
It was earlier suggested that the report would be delayed after the Met Police confirmed it was investigating a number of the events concerned.
However, the Mirror reports that sources have told the paper Downing Street is now expecting to receive the report at some point on Tuesday night (January 25).
If No 10 receive the report on Tuesday night, it would likely be published tomorrow.
There were concerns that civil servant Ms Gray would have to pause parts of the inquiry while the police investigated allegations of rule-breaking, which would either delay the publication of the final report, or split it in two with only some parts published ahead of the police findings.
It is now understood that after talks between the Cabinet Office and Scotland Yard, it has been decided that Ms Gray's report can still be released.
A government source told the Mirror that No 10 getting hold of Sue Gray’s report "could happen as quickly" as Tuesday if there are no major obstacles.
If so, the report could be published on Wednesday just before PMQs, the source said.
According to BBC political correspondent Nick Eardly, there is a 'working plan' for the PM to make a statement on the report after PMQs, provided it is sent to No 10 before then.
He said opposition sources have asked for advance copies of the report to give them time to study it before the PM makes a statement.
What will happen after the inquiry?
Downing Street has insisted that the PM will accept the findings of the inquiry.
Ultimately, it will be down to elected politicians to make the final judgement on whether the prime minister should resign.
A vote of no confidence could be triggered if just 54 Tory MPs write to Sir Graham Brady, chair of the 1922 committee, calling for a leadership contest.
