Photos published on Monday show the prime minister raising a glass with at least six others in front of a table littered with open bottles of wine and liquor.
They are said to have been taken at a leaving do for his former top spin doctor, Lee Cain on 13 November 2020.
Rules at the time only allowed two people from different households to mix indoors in non-work circumstances.
Details of the gathering surfaced as part of the first ‘partygate’ claims made the following year, heaping pressure on Mr Johnson to explain his involvement.
At a Prime Minister’s Questions held in Parliament on December 8 – just over a week after the first allegations were published – Labour’s Catherine asked him: ‘Can the prime minister tell the House whether there was a party in Downing Street on 13 November?’
He replied: ‘No, but I’m sure whatever happened the guidance… and the rules were followed at all times.’
Mr Cain’s leaving party, which took place eight days after England was put under another lockdown, was one of several gatherings looked at by a now-completed Metropolitan Police investigation.
Sources cited in multiple news reports suggested Mr Johnson had instigated the event, pouring drinks and encouraging urging staff to ‘let off steam’, though this was not corroborated by the police.
The gathering took place on the same day that his former chief aide Dominic Cummings resigned, ending a reported power struggle between Mr Cummings and the PM’s wife, Carrie Johnson.
The PM was not fined over the incident, only receiving a £50 fixed penalty notice for attending a surprise birthday party thrown for him in Number 10 in June 2020.
Earlier this month he said that he had not been contacted by police about Mr Cain’s leaving party at all.
There was widespread speculation that a number of previously unseen pictures would be published after senior civil servant Sue Gray completed her report into the scandal.
Both the report, due to be published as early as Tuesday, and any new images could be extremely damaging for the prime minister.
Deliberately misleading Parliament is conventionally a resigning matter for any MP, and the pictures could bolster accusations that Mr Johnson has done so.
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