Met Police Commissioner Cressida Dick has announced Scotland Yard is now investigating several events in Whitehall and Downing Street for potential breaches of Covid laws.
She said while it is not standard procedure for the Met to investigate retrospective breaches, some occasions called for it - insisting the force would "police without fear or favour".
Downing Street has indicated Boris Johnson is willing to be interviewed by police and could hand over diaries, phone records and other evidence.
It raises the prospect of junior Downing Street staff or even the Prime Minister being handed fixed penalty notices by police if they are found to have broken the law.
A police probe could take weeks or months, much longer than the Sue Gray civil service report on parties which is set to be published as soon as today.
Here we take a look at who detectives might want to interview for their 'partygate probe' - and what they might ask them.
Boris Johnson
The PM is said to have been present at half a dozen No 10 bashes including the BYOB garden party, the Christmas quiz, his birthday party in the Cabinet room and at least three staff leaving dos.
He is also said to have told staff to "let off steam" during their regular 'wine-time Fridays' throughout lockdown.
Scotland Yard needs to challenge the PM's claims that he thought he was attending work events.
Although it potentially contains sensitive information, they will need access to his phone - including his previous one - so that any text or Whatsapp messages inviting staff, family and friends to lockdown-busting events are uncovered.
They will also try to pin him down on his detailed comings and goings to the multiple events.
No10 have indicated they would hand over the PM’s phone records - but not whether that would include his old phone, which he binned after it emerged his number was online.
Carrie Johnson
The PM's wife is said to have arranged his surprise birthday party in the Cabinet room of No 10, leading staff in a chorus of happy birthday and providing a cake and M&S picnic.
Sources also claim she was at the BYOB garden party drinking gin and tonics - despite her husband claiming he thought it was a work event - and even invited along two friends who worked in other parts of Whitehall.
The same two friends are among those said to have been invited to the Downing Street flat during lockdown - although No 10 said this was a work event.
Sources told the Mirror that Carrie was "addicted" to throwing regular lockdown parties - but this was fiercely denied by No 10.
Detectives may want to ask Mrs Johnson - and her friends - for details and check whether her Whatsapp messages corroborate her account.
Martin Reynolds
The PM's principal private secretary, the most senior civil servant in his office, is at the heart of one of the most explosive party allegations.
He was the author of the email inviting up to 100 Downing Street staff to a BYOB garden party on May 20, 2020.
Mr Johnson claimed that he thought this was a work event - but at least two senior aides are said to have warned him not to go as it was against the rules.
The Met will want to know whether the PM instructed Mr Reynolds to send the invite and - when he was told it was a bad idea - whether he passed that warning on to Mr Johnson.
As well as accessing his emails, officers will want him to provide a full account of the conversations that day - as well as quizzing him about other gatherings he attended like the PM's surprise birthday bash in the Cabinet room.
Dan Rosenfield
Mr Johnson's chief of staff has been accused by many Tory MPs of allowing a culture at No 10 where staff believed it was acceptable to flout the lockdown rules the rest of the country was living by.
Mr Rosenfield, who took over as chief of staff in January 2021, is understood to have been aware of, if not actually present at, a number of the gatherings.
Detectives will want to know whether he attended any of the bashes in person, and as the PM's top aide, if he knew about any alleged gatherings in the Downing Street flat.
They will also be interested in any conversations he may have had with the PM about social events throughout lockdown - and if he has any emails or Whatsapp messages that show the PM was involved or not.
He may also have been aware of get-togethers in other parts of the building.
Jack Doyle
The PM's director of communications is the only member of the Government's top media team that organised several social events at No10 in the run-up to Christmas 2020 who is still in place.
He was allegedly present at the first Christmas bash on December 18, 2020, revealed by the Mirror, as well as other events including the PM's birthday party.
His team held 'wine-time Fridays' throughout lockdown and even bought a chiller to keep bottles of fizz and beer cool.
His predecessor James Slack, whose leaving do was on the eve of Prince Phillip's funeral, left No 10 in Spring 2021.
Allegra Stratton, the former Press Secretary, quit after video emerged of her laughing about the December 18 Christmas party.
Ex-communications chief Lee Cain left in Autumn 2020.
However, the Met may decide to interview them all about their recollections, as well as ask them for access to their emails and any internal No 10 Whatsapp groups.
Dominic Cummings
Boris Johnson's closest aide, turned fiercest critic, first made the explosive claims about the BYOB garden party on May 20, 2020, and subsequently claimed he had warned the PM it would be against the rules.
He has repeatedly claimed that photos exist of other events at No 10 - so the police may want to ask if he actually has any of these in his possession.
He has said he would "swear under oath" about the Downing Street parties - and that senior advisors still in No 10 would do the same.
However, he ultimately refused to be interviewed by Sue Gray, claiming the PM would try to distort his testimony, and that he would submit written evidence instead.
The police are unlikely to accept a similar excuse and will want to speak to him in person - as well as asking him to hand over any Whatsapps and pictures he may have in his possession.
