Boris Johnson and his wife Carrie, and chancellor Rishi Sunak, have been notified that they will be issued with fines for breaking lockdown rules.
The fixed penalty notices are the result of a Metropolitan Police investigation into parties in Downing Street and Whitehall in 2020 and 2021.
Labour immediately called for both to the PM and chancellor resign while Liberal Democrat leader Sir Ed Davey called for parliament to be recalled for a vote of confidence in Mr Johnson.
Earlier, the Met disclosed that it had now referred more than 50 cases for fixed penalties as a result of the probe, called Operation Hillman.
A Downing Street spokesman said: “The prime minister and chancellor have today received notification that the Metropolitan police intend to issue them with fixed penalty notices.”
Labour leader Sir Keir Starmer said: “Boris Johnson and Rishi Sunak have broken the law and repeatedly lied to the British public. They must both resign.
“The Conservatives are totally unfit to govern. Britain deserves better.”
In January, the Met launched an investigation into 12 events where restrictions may have been breached.
Read more: Everything you need to know about the police investigation of partygate
Officers required around 100 Downing Street and Whitehall staff members including the PM and chancellor to fill out formal legal questionnaires as part of the investigation.
In late March, the force confirmed detectives had begun interviewing key witnesses on details related to the questionnaire responses.
The prime minister received a questionnaire on 11 February. It is thought he attended as many as six of the events under scrutiny.
Police issued the first 20 fines on 1 April to people who attended a leaving do for a Number 10 official on 18 June 2020 in the Cabinet Office building at 70 Whitehall.
Some of those fines were also handed out to people who attended two events on 16 April 2021, the day before Prince Philip’s funeral.
Fixed penalty notices do not amount to a criminal conviction, but they can result in a court summons and criminal prosecution if they are not paid.
The 12 events investigated by the police took place over eight separate dates and include a “bring your own booze” event in the Downing Street garden on 20 May 2020, organised by the PM’s principal private secretary Martin Reynolds. Another was a surprise birthday gathering for Mr Johnson in the cabinet room in June 2020, attended by the chancellor.
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.
Scotland Yard launched its investigation following the conclusion of the internal government inquiry led by senior civil servant Sue Gray.
A restricted version of her report was published at the end of January, but many details were removed so as not to compromise the police investigation.
It is unclear whether the full report will now be published. The PM’s official spokesman has previously said discussions were needed “with the Met and others about what is suitable”.