Meanwhile, the leaving party for two No 10 employees in Jan 2021 broke the rule banning individuals from meeting people socially with anyone outside of their household.
How much will the Prime Minister have to pay?
Fines for breaching Covid lockdown restrictions have ranged from £50 to £10,000, depending on the breach and how long it takes the recipient to pay.
The first batch of FPNs issued by the Met to Downing Street staff last month were £50.
However, for members of the public the fines originally started at £60, but were reduced to £30 if paid within 14 days. Second-time offenders could be fined £120, with this doubling for further offences.
But by the latter stages of the pandemic fines were increased to £200, reduced to £100 if paid within 14 days.
What happens now?
Boris Johnson will pay the fine he was issued on Tuesday.
However, he could be issued further fines if the Met Police find that he attended other illegal gatherings. He will then have the option to challenge those fines on an individual basis in court should he see fit.
Politically, the Prime Minister could face a challenge to his leadership if Sir Graham Brady, the chairman of the 1922 committee, starts to receive letters calling for a vote of confidence.
If 54 letters are received, a vote would be triggered for when Parliament returns after Easter recess. If Mr Johnson loses then a leadership election would take place.
Does the Prime Minister have to accept a fine?
The Prime Minister can challenge his fixed penalty notice, which would involve taking the case to a Magistrates’ Court if he believes the penalty should not have been issued.
Does he have a criminal record?
No. A fixed penalty notice is an administrative alternative to prosecution which does not appear on a criminal record. If left unpaid and if the Prime Minister loses his appeal, it can amount to a criminal offence.
Has any other PM ever done something similar?
Mr Johnson is the first known Prime Minister to have broken the law whilst in office, and the first who is known to have received a fixed penalty notice.
When was the last time a PM was involved in a criminal investigation?
When Sir Tony Blair was in No 10 he was questioned by police – but not under caution – over cash-for-honours allegations.
In July 2007 the former prime minister, who was questioned three times by officers, welcomed the decision not to prosecute anyone over the claims.
The police probe into the alleged sale of peerages ended in an announcement from the Crown Prosecution Service that there was insufficient evidence against anyone to secure a conviction in court.