Twenty fines could be issued in the coming hours over events held in Downing Street and Whitehall during lockdown, Sky News understands.
The fines will be issued by detectives investigating the “partygate” scandal.
It is believed these punishments are only the first, and others are expected as more than 100 people were handed questionnaires by the Metropolitan Police as part of the investigation – named Operation Hillman.
Whitehall sources suggested the fines could come as early as Tuesday, but said the Met may change its timetable.
On Tuesday, government minister Will Quince acknowledged that “considerable upset” had been caused over events that “shouldn’t have happened” but was tight-lipped when asked if he thought the prime minister should resign if fined.
Mr Quince, who is minister for children and families, told Sky News: “That’s a hypothetical question, you wouldn’t expect me to answer that.”
The Met launched an investigation at the end of January into 12 alleged events that happened on eight dates during England’s lockdowns in 2020 and 2021 to see if COVID regulations were broken.
One of the events that created an uproar was a gathering in the Downing Street garden at the height of the UK’s first national lockdown on 20 May 2020, with a leaked email from one of the prime minister’s chief aides inviting staff to a “bring your own booze” event.
Mr Johnson has said he would reveal if he had been fined.
The PM is known to have attended at least two other gatherings under investigation – one on 19 June 2020 in the Cabinet Room for his birthday and another on 13 November 2020 for a special adviser’s leaving do.
Several Conservative MPs had called for Mr Johnson to resign over the parties, but some have changed their stance since the war in Ukraine began just over a month ago.
Questionnaires sent to suspected attendees
All those believed to have attended alleged lockdown-busting parties in Westminster were sent an email questionnaire asking for their account and explanation of their participation.
The Met Police announced in early February its officers were beginning to contact people believed to have been involved in the 12 events under its investigation.
Mr Johnson received one on 11 February and, like everyone else, had seven days to respond.
The Met Police said the document has formal legal status and must be answered truthfully.
It said no further action would be taken against people who pay the fine and they will consider whether to pursue the matter in court if the fine is disputed.
Scotland Yard’s investigation is the second inquiry to take place into claims Number 10 officials – including the prime minister – were partying while England was under COVID-19 restrictions.
Sue Gray inquiry
Senior civil servant Sue Gray led a Whitehall probe into the allegations that led many to call into question Mr Johnson’s political future.
However, a full report was not published at the end of January because the Met Police made a U-turn after saying it would not investigate the events.
Ms Gray’s report found there were “failures of leadership and judgement” regarding the gatherings between May 2020 and April 2021.
She was looking at 16 events, with the police examining all but four of those.
The PM said a full version of her report will be published once the police have finished their investigation.