When asked in the Commons in December 2021 whether there had been a party in No 10 on that occasion, the Prime Minister replied "no," adding that he was confident the rules were followed at all times.
Johnson was photographed with a drink in his hand alongside a table piled with food and wine bottles, and the Metropolitan Police has been questioned as to why he was not penalised in relation to the incident.
At least eight additional people were in the room at a time when social mingling was prohibited - except for meeting one person outside - and at least one person received a fixed penalty notice in connection with an event on the same date.
So how did Johnson escape another fine, and how much would he have had to pay had he been slapped with a second Fixed Penalty Notice (FPN)?
Here is everything you need to know.
How much was Boris Johnson’s fixed penalty notice fine?
The first wave of Partygate fines were issued by the Metropolitan Police, with 20 junior civil servants given £50 fines apparently delivered by email.
The fixed penalty notices were thought to relate to a leaving party held in the Cabinet Office on 18 June 2020 to mark the departure of a private secretary, Hannah Young.
Boris Johnson said it “did not occur” to him that the gathering in the Cabinet Room on 19 June 2020 to mark his 56th birthday was a violation of coronavirus rules, but that he “now humbly accepts” he did breach Covid-19 laws.
Both the Prime Minister and Rishi Sunak received fixed penalty notices, paid at a reduced rate of £50, down from £100, because they were paid within a 14-day period.
Rishi Sunak, meanwhile, said: “I deeply regret the frustration and anger caused and I am sorry.”
Carrie Johnson, the Prime Minister’s wife, was also issued with a fine which has been paid.
Speaking on BBC Breakfast, Transport Secretary Grant Shapps said the Prime Minister “has paid the fixed penalty notice of £50 and apologised.”
Given the wealth of both the Johnsons and Sunak, it is unlikely that a £50 fine leaving their bank accounts would have caused too much financial hardship.
What is a fixed penalty notice?
Fixed penalty notices are a low-level sanction for breaking the law, and mean a fine, which needs to be paid or contested.
If someone chooses to contest the fine, the police will then review the case and decide whether to withdraw the fine or take the matter to court.
Fixed penalty notices for breaching Covid regulations do not lead to a criminal record, and are somewhat equivalent to a minor speeding offence.
Each of the Met Police's decisions to issue fixed penalty notice in relation to Partygate will go to Acro, the criminal records office for England and Wales, which processes the payments required from each person fined
That means there is an official record on the system - but it is not a criminal record that must be disclosed.
The amount that needs to be paid for a Fixed Penalty Fine in relation to Covid-rule breaking doubles with each one issued.
Why wasn’t Johnson fined a second time?
Daisy Cooper, the Liberal Democrat deputy leader, has written to the Independent Office for Police Conduct (IOPC), requesting that it investigate the Met's “Operation Hillman” investigation into events in No 10 and Whitehall.
Mayor of London Sadiq Khan has said the Met should explain why Boris Johnson was not penalised for attending a party where at least one other person was fined.
Cabinet minister Grant Shapps believes the Met does not need to explain why the Prime Minister was not penalised over the 13 November 2020 leaving party.
Shapps said he was "angry" to see photos of Johnson holding a drink, but suggested that he was not penalised since he departed the event "pretty quick."
Discussing the pictures, the Transport Secretary told BBC Radio 4’s Today programme: “I don’t think the police should provide running commentaries, no.”
Shapps had earlier denied Johnson was partying at the leaving party during the second lockdown.
He told Sky News: “The question is, was he down there partying? No, clearly not. He’d gone by to say thanks and raise a glass to a colleague who was leaving.”
But Conservative MP David Simmonds has demanded Johnson explain how the photographs do not depict him drinking at a rule-breaking party.