Boris Johnson's Covid lockdown fine relates to a birthday party thrown for him in the first lockdown - while indoor gatherings were illegal.
No10 today confirmed his £100 fine - halved to £50 if paid within 14 days - relates to a gathering on 19 June 2020 where an ally infamously claimed he was "ambushed by cake".
Staff are said to have sung Happy Birthday while it was still illegal to sing hymns at Covid victims' funerals.
It comes after the Mirror was told police had a photograph of Boris Johnson holding a can of an Estrella beer at the bash - and raised it to the camera in a toast.
The bombshell image was thought to have been taken by Mr Johnson’s official taxpayer-funded photographer who was documenting the event.
The Prime Minister was pictured standing next to Chancellor Rishi Sunak in No10's Cabinet room where the event was allegedly arranged by his wife Carrie Johnson.
All three were fined today, among 50 lockdown breach fines issued so far.
The Met Police said extra fines could still follow - raising the bombshell prospect that Boris Johnson could be handed more fines over further events as the investigation proceeds.
A No 10 spokesman said: “The Met Police have now explained that the FPN issued to the PM will be in relation to the following incident:
“On 19th June 2020 at the Cabinet Room 10 Downing Street between 1400 and 1500 you participated in a gathering of two or more people indoors in the Cabinet Room at 10 Downing Street.”
It comes after 'lying' Boris Johnson and Rishi Sunak were told to resign as police issued them both with Partygate fines - along with the PM's wife Carrie.
No10 made the bombshell admission that cops have decided the Prime Minister, his wife and his Chancellor all broke their own lockdown laws.
The Prime Minister - the first found by police to have broken the law - faced demands to recall Parliament for a vote of no confidence weeks before the May 5 local elections.
But Tory MPs who previously demanded his resignation rallied round and said now is not the time due to Russia's war in Ukraine.
It came as Scotland Yard dished out another 30 fines for lockdown-breaching parties in Downing Street and on Whitehall in 2020 and 2021 - bringing the total to 50 so far.
Mr Johnson is accused of misleading Parliament four and a half months ago when he vowed solemnly: "All guidance was followed completely in No10".
Carrie Johnson had also vehemently denied a party took place in the Downing Street flat, while Rishi Sunak told MPs in December: "I did not attend any parties."
Labour leader Keir Starmer said: "Boris Johnson and Rishi Sunak have broken the law and repeatedly lied to the British public. They must both resign."
This breaking news story is being updated.
