Media have reported that at least 15 gatherings took place at Johnson's 10 Downing Street residence or in other government departments between May 2020 and April 2021, when COVID-19 rules limited how many people could meet socially.
Sue Gray, a senior civil servant, is leading an inquiry. She has not said when her findings will be published, but they had been expected later this week.
Below is a timeline of the party reports and what Johnson has said about them:
Nov 30 - First lockdown Christmas party report
The Mirror newspaper reports that Christmas parties took place in government offices, including Johnson's own in Downing Street, in December 2020, when such gatherings were banned.
Later media reports include a photo of Johnson hosting an online Christmas quiz for his staff, seated alongside an official draped in tinsel.
Dec 1 - Johnson denial
Asked by the leader of the opposition Labour Party about reports of a December 2020 Christmas party, Johnson tells parliament: "All guidance was followed completely in No. 10."
Dec 7 - Video of staff joking about party
ITV News publishes a leaked video showing Johnson's staff laughing and joking during a mock news conference over how to explain a gathering in Downing Street.
In the video, Allegra Stratton, then Johnson's press secretary, says: "This fictional party was a business meeting, and it was not socially distanced."
Dec 8 - Johnson apologises, aide resigns
Johnson tells parliament: "I apologise unreservedly for the offence that (the Stratton clip) has caused up and down the country, and I apologise for the impression that it gives.
"I have been repeatedly assured since these allegations emerged that there was no party and that no COVID rules were broken... if those rules were broken, there will be disciplinary action for all those involved."
Dec 9 - Details of inquiry published
The government says Britain's top civil servant, Simon Case, will lead an inquiry into three alleged gatherings. After additional media reports, it is broadened to cover further gatherings.
Dec 17 - Lead investigator steps down
Case steps down after reports that an event was held in his own office. The government says the investigation will be concluded by Gray.
Dec 19 - Garden party photo published
The Guardian newspaper publishes a photograph of Johnson and more than a dozen others drinking wine in the Downing Street garden that it said was taken during lockdown on May 15, 2020.
Dec 20 - Johnson says garden party was work
"Those were people at work talking about work," Johnson said when asked about the Guardian picture.
Jan 10 - Party invitation from Johnson's aide published
ITV publishes an email sent by Johnson's Principal Private Secretary Martin Reynolds to over 100 Downing Street employees on May 20, 2020, inviting them "after what has been an
incredibly busy period... (to) some socially distanced drinks in the No10 garden this evening ... and bring your own booze!"
ITV said around 40 staff gathered, including Johnson and his partner Carrie, even though at the time social mixing between households was limited to two people outdoors.
Jan 12 - PM says he attended lockdown gathering
Johnson tells parliament he attended a gathering on May 20, 2020, in the Downing Street garden and apologises.
"When I went into that garden... to thank groups of staff before going back into my office 25 minutes later to continue working, I believed implicitly that this was a work event," he said. His press secretary says he did not see Reynolds' email.
The Times newspaper cites Conservative lawmakers saying Johnson later told them he did not believe he had personally done anything wrong and said "we're taking hits for something we don't deserve".
Jan 13 - Police decline to investigate gatherings
British police say they will not investigate the Downing Street gatherings unless Gray's inquiry finds evidence of potential criminal offences.
Jan 14 - PM apologises to Buckingham Palace
Johnson's office apologises to Queen Elizabeth after it emerged staff partied late into the night in Downing Street on the eve of her husband Prince Philip's funeral in April 2021, when mixing indoors was banned. Johnson was at his country residence that day, his spokesman says.
One of those parties was a leaving event for James Slack, a former director of communications, who apologises and says the gathering "should not have happened at the time that it did".
Jan 17 - Former aide says Johnson knew about May 2020 party
Dominic Cummings, a former senior adviser to Johnson, says he is willing to "swear under oath" the prime minister knew about the May 20, 2020, party.
Cummings says he and at least one other adviser told Reynolds the drinks should not go ahead and that this warning was sent via email. According to Cummings, Reynolds said he would check with Johnson if he was happy for the drinks to go ahead, and he is sure Reynolds did so.
Jan 24 - Report of Johnson 2020 birthday gathering
ITV News says up to 30 people attended a Jun 19, 2020, birthday event for Johnson at 10 Downing Street at which he was presented with a cake while his wife led staff in a chorus of happy birthday. Johnson's office confirm staff gathered briefly after a meeting but dispute it was a party.
Jan 25 - Police say they are now investigating
London's Metropolitan Police say have begun investigating "a number of events" that took place in Downing Street and other government departments after receiving information from Gray's inquiry.
The Cabinet Office says Gray's inquiry will continue, although media report the publication is likely to be delayed until the police investigation is concluded.