A clip obtained by ITV news shows a press conference rehearsal three days after the party is reported to have been held.
Downing Street has repeatedly denied any such gathering took place in December last year, when lockdown restrictions forbade people from meeting in large groups. Boris Johnson has insisted all Covid-19 rules were followed.
But in the leaked footage, staff can be heard joking and asking about a “Christmas party” and “cheese and wine evening” that was “not socially distanced”.
Commenting on the leaked footage, SNP Westminster Leader Ian Blackford MP said: "This video is damning and leaves the Prime Minister with a lot to answer for.
"When every individual up and down the country was told to stay at home in order to protect the NHS, the Tories were instead having a Christmas party which, as a senior staff member has now confirmed, was ‘not socially distanced’.
"If this is true, then the Prime Minister's position is untenable and he must remove himself from office immediately."
Mr Johnson’s then spokesperson Allegra Stratton can be seen in the leaked clip answering practice questions at the podium of the Number 9 Downing Street press room.
Ed Oldfield, a No.10 adviser, is heard asking: “I’ve just seen reports on Twitter that there was a Downing Street Christmas party on Friday night, do you recognise those reports?”
“I went home”, Ms Stratton replies, laughing. She then pauses, apparently considering what the answer should be.
Mr Oldfield asked: “Would the Prime Minister condone having a Christmas party?”
Ms Stratton laughed again and responded: “What’s the answer?”
Other staff in the room are then heard chipping in to jokingly offer answers. One says: “It wasn’t a party, it was cheese and wine.”
Ms Stratton asked “is cheese and wine all right?” and someone can be heard saying “no … was joking”.
Ms Stratton joked “it was a business meeting” before adding: “Is this recorded? This fictional party was a business meeting and it was not socially distanced.”
ITV reported the clip came from a rehearsal carried out on December 22 – three days after one of the Christmas gatherings is said to have been held where staff brought alcohol, food and secret Santa gifts.
The Mirror revealed last week that 40 to 50 people gathered for the event on December 18.
The next day the Prime Minister ordered people to stay at home for Christmas, preventing millions from visiting loved ones.
One source told ITV News: “We all know someone who died from Covid and after seeing this all in the papers I couldn’t not say anything. I’m so angry about it all, the way it is being denied.”
The Prime Minister instead insisted no rules were broken as it was reported that staff wore festive jumpers and were asked to bring “secret Santa” gifts.
Mr Johnson’s official spokesman has insisted “there was not a party”, but the Prime Minister declined to characterise the event during a visit to a prison in London yesterday.
"What I can tell you is that all the guidelines were observed, continue to be observed.” he told broadcasters when questioned at HMP Isis as the UK Government tried to portray itself as tough on crime.
Asked if he investigated personally, Mr Johnson said: “I am satisfied myself that the guidelines were followed at all times.”