Boris Johnson is believed to have attended at least six gatherings being probed by a police inquiry into lockdown parties in Whitehall and Downing Street.
Scotland Yard is investigating 12 rule-breaking events after U-turning in January on its initial refusal to probe Partygate allegations.
The force confirmed on Tuesday that 20 fixed penalty notices are being referred to the ACRO Criminal Records Office as part of the first wave of fines.
The Prime Minister received formal questionnaire from the Metropolitan Police, along with other high profile figures including Chancellor Rishi Sunak and civil service chief Simon Case.
But Mr Johnson is not expected to be among those in the first wave, as sources believe his case is at the "bottom of the pile" due to its complicated and controversial nature.
Police received 500 documents and 300 photos for their inquiry, which has seen 100 questionnaires sent out to those believed to be involved.
Some events Mr Johnson attended in person are not being probed by detectives, including a gathering in the Downing Street garden with cheese and wine in May 2020.
But he faces being fined for breaking Covid rules for several parties that he was at himself.
The Mirror led the way in revealing a number of parties and others have since been uncovered by a string of media outlets.
These are the events Prime Minister is believed to have attended.
Events being looked at by police
May 20 2020: BYOB garden party at Downing Street
The PM’s top aide Martin Reynolds invited 100 No10 staff to “make the most of the lovely weather ” and “bring your own booze" in a leaked email.
Boris Johnson later admitted spending 25 minutes at the party, claiming he thought it was a work event.
Some 40 guests mingled outside with long tables laden with drink, crisps, sausage rolls and other picnic food. Sources branded it "a proper booze-up.”
Just 55 minutes earlier, Cabinet Minister Oliver Dowden told a press conference: “You can meet one person outside of your household in an outdoor, public place - provided that you stay two metres apart".
June 19 2020: Boris Johnson's birthday
Downing Street has admitted staff "gathered briefly" in the Cabinet Room at a surprise birthday celebration for the Prime Minister organised by his wife.
More than 30 officials gathered for the PM's birthday bash in the Cabinet Room for up to half an hour in the afternoon, according to ITV News.
Mrs Johnson led the staff in a chorus of happy birthday while the PM was presented with a cake, before tucking into picnic food from M&S.
Chancellor Rishi Sunak admitted he was also in the room but claimed he was there for a Covid meeting.
He told the BBC: "You’re asking me about something that happened almost two years ago. I went to that Cabinet Room, as I did many other times for Covid meetings."
Lulu Lytle, the interior designer behind lavish renovations of the Downing Street flat, also briefly attended.
Later that evening, the PM and his wife were said to have hosted family friends in their residence upstairs in an apparent further breach of the rules. Downing Street denied these claims - saying the PM hosted a small number of family members outside instead.
A No 10 spokesman said: "A group of staff working in No 10 that day gathered briefly in the Cabinet Room after a meeting to wish the Prime Minister a happy birthday. He was there for less than ten minutes.”
November 13 2020: Downing Street flat do
The most dangerous allegations facing the PM are claims of parties in the flat he shares with his wife Carrie above No 11 Downing Street.
Downing Street and Mrs Johnson have always denied this.
But sources told the Mirror No10 aides “drank their socks off" at an alleged gathering on November 13 - the night Mr Cummings left.
Staff reported hearing "loud music pumping out of the flat" late into the night, which could reportedly be heard from nearby St James' Park.
The Mail on Sunday even described it at the time as a “victory party” celebrating the departure of Mr Cummings - a rival of Mrs Johnson's - and said Abba's 'The Winner Takes it All' could be heard playing.
Mr Cummings has claimed there was a party in the flat on November 13 and “other flat parties” that should be investigated.
A spokeswoman for Mrs Johnson dismissed the claims as "total nonsense”, adding: "Mrs Johnson has followed coronavirus rules at all times and it is categorically untrue to suggest otherwise.”
England was in a national lockdown and that day the UK recorded 436 Covid deaths - the second-highest daily total since May.
November 13 2020: Leaving bash for spin doctor Lee Cain
Boris Johnson gave a leaving speech to say farewell to his spin doctor Lee Cain, who left No 10 alongside close ally Dominic Cummings.
Sources told the Mirror a relatively small number of people gathered for drinks in No10’s ‘round table room’ to mark two top aides’ departure.
Mr Cain posed for a picture alongside Mr Johnson that night with the pair donning “Get Brexit Done” boxing gloves.
The PM gave a “shambolic” speech at the low-key affair, with everyone gone by about 9pm, sources suggested.
December 17 2020: No 10 leaving bash
Mr Johnson is understood to have attended a leaving do for his defence adviser in the run-up to Christmas 2020.
The PM gave a speech at a gathering to mark the departure of defence aide Captain Steve Higham.
It is understood the PM was “there for a few minutes to thank him for his service” as a “small number of No10 staff briefly said goodbye”.
The Ministry of Defence declined to comment.
January 14 2021: Prosecco-fuelled leaving drinks
A boozy gathering was held in No 10 to mark the departure of two private secretaries, where guests drank prosecco.
Reports have suggested the Prime Minister attended the leaving event, which was for a senior civil servant in the Department for Digital, Culture, Media and Sport, according to the Telegraph.
Police might investigate this event
December 15 2020: No 10 online Christmas quiz
The PM was pictured hosted a virtual Christmas quiz, with two staff by his side and staff huddled around computers in Downing Street while enjoying drinks bought at a local Tesco Metro, sources told the Mirror.
London was then under Tier 2 regulations banning social mixing between households.
Official guidance also stated: “You must not have a work Christmas lunch or party, where that is a primarily social activity and is not otherwise permitted by the rules in your tier.”
The Met initially said it was not investigating this event.
But the force announced it was reviewing its decision after the Mirror publish another bombshell picture from the event.
The image shows Mr Johnson and staff preparing for the quiz, with an open bottle of bubbly and a bag of crisps visible in the shot.
The force said: "The Met previously assessed this event and determined that on the basis of the evidence available at that time, it did not meet the threshold for criminal investigation.
"That assessment is now being reviewed."
Events NOT being looked at by police
May 15 2020: Cheese and wine in the No 10 garden
Around 20 people including Boris and Carrie Johnson were pictured gathered in the sunshine in the Downing Street garden.
England was still in its first lockdown, with gatherings in gardens banned completely and a two-person limit on meeting in the park.
Mr Johnson insisted it was a meeting of “people at work, talking about work” despite the fact he was talking to his fiancee.
Others in the photo included the then-Health Secretary Matt Hancock and the PM’s top aides Martin Reynolds and Dominic Cummings.
November 27 2020: Another special adviser leaves
Around 40 to 50 people crammed into a room in No10 for a leaving do for top aide Cleo Watson, sources told the Mirror.
Guests allegedly had drinks late into the night - and Boris Johnson popped in to give a speech.
No10 refused to comment on the claims when first reported by the Mirror.
Dominic Cummings, an ally of Ms Watson, claimed this was a "red herring".
He said: "There was no party on Fri 27/11. Red herring. A staff member left their job. Walked to press office to say bye, PM bumbled in & started babbling, everyone embarrassed, dispersed."
