Police will hand out 20 fines to people who attended lockdown-busting parties in Downing Street and Whitehall, Scotland Yard has said.
Detectives have been investigating 12 gatherings during the pandemic, including as many as six events Boris Johnson is believed to have attended.
The Prime Minister is not expected to be among those included in the first wave of fines, which is understood to contain the most straight-forward cases.
Downing Street said the Metropolitan Police has not informed No 10 that Boris Johnson is among the people referred to the ACRO Criminal Records Office for fixed penalty notices.
It is not thought Mr Johnson is among those set to receive fines, and the Prime Minister's official spokesman said: "We've said we'll update if that were to occur, but our position has not changed."
Mr Johnson is believed to be at the "bottom of the pile" as his case is politically explosive and complicated by the fact that Downing Street is his home as well as his workplace.
Police will not name those who receive fixed-penalty notices - but Downing Street has committed to disclosing if Mr Johnson is personally fined.
The bombshell announcement makes it explicit that Covid rules were broken during Partygate - despite repeated attempts by Mr Johnson, No10 and people in Government to wriggle out of censure.
It comes months after the Mirror first broke the story of lockdown-flouting Christmas parties held as the rest of the country lived under strict Covid rules.
In a statement, the Metropolitan Police said: "We will today initially begin to refer 20 fixed penalty notices to be issued for breaches of Covid-19 regulations"
"The investigation into allegations of breaches of COVID-19 regulations in Whitehall and Downing Street has now progressed to the point where the first referrals for fixed penalty notices (FPN) will be made to ACRO Criminal Records Office."
"We are making every effort to progress this investigation at speed and have completed a number of assessments.
"However due to the significant amount of investigative material that remains to be assessed, further referrals may be made to ACRO if the evidential threshold is made."
More than 100 police questionnaires had been sent to people believed to have attended lockdown gatherings, including Mr Johnson, who took private legal advice on how to respond, and Chancellor Rishi Sunak.
The events included a notorious "bring your own booze" bash in the Downing Street garden in May 2020, a surprise get-together for Mr Johnson's birthday in June 2020, and a party in the PM's No11 flat in November 2020.
Mr Johnson previously admitted he was at the BYOB event for 25 minutes - but he tried to claim to MPs he believed it was a work event.
Downing Street said staff "gathered briefly" in the Cabinet Room to mark the Prime Minister's 56th birthday.
Scotland Yard initially resisted investigating multiple allegations of rule-breaking before U-turning in January.
Labour Deputy Leader Angela Rayner called on the Prime Minister to resign.
"After over two months of police time, twelve parties investigated and over a hundred people questioned under caution, Boris Johnson’s Downing Street has been found guilty of breaking the law," she said.
"The culture is set from the very top. The buck stops with the Prime Minister, who spent months lying to the British public, which is why he has got to go.
"It is disgraceful that while the rest of the country followed their rules, Boris Johnson's government acted like they did not apply to them.
"This has been a slap in the face of the millions of people who made huge sacrifices."
Families who lost lost loved ones to the virus said the Prime Minister was "pouring more salt on the wounds of those who have suffered so much" by failing to resign.
Hannah Brady, spokesperson for Covid-19 Bereaved Families for Justice, said: "A year ago today, bereaved families from across the UK drew the first hearts on the National Covid Memorial Wall. One heart for every life lost to the virus.
"A year down the line, far from offering closure, the Prime Minister's team are being fined for breaking their own rules regularly and blatantly. The same rules that families across the country stuck with even when they suffered terribly as a consequence.
"It’s crystal clear now that whilst the British public rose to the challenge of making enormous sacrifices to protect their loved ones and their communities, those at 10 Downing Street failed.
"Frankly, bereaved families have seen enough. The PM should have resigned months ago over this. By dragging it out longer all he is doing is pouring more salt on the wounds of those who have already suffered so much."
