The Mirror obtained the image of assistants crowding around a buffet.
They alleged the December 14 gathering was organised by Shaun Bailey's London mayoral campaign crew. At the time, it was restricted to Tier 2.
Top Tory Shaun Bailey and wealthy Tory donor Nick Candy are shown in the shot.
Last year, Tier 2 limits limited outside visits to six people from separate houses. They could only socialise with members in their household or support bubble and were instructed to keep social distance from others.
Mr Bailey resigned as chair of the London Assembly's police and crime committee today afternoon, the publication reports.
Four members of his campaign crew had previously been disciplined over the party in the basement of the Conservative HQ in Westminster.
According to hacked emails, arrangements for certain colleagues to attend a Christmas quiz in person were established at least four days prior.
On the day, the Head of HR at Number 10 urged those present to ‘go out the back'.
‘All personnel will be forced to leave the workplace immediately after the quiz,' according to an email delivered the previous Friday.
But several allegedly drank and spoke after the incident. Work parties were outlawed.
Professor Quiz Whitty, Next Slide Please, The 6 Masketeers and Hands, First Place.
Last Thursday, a photo of Boris Johnson conducting the quiz prompted new questions about alleged rule-breaking in Downing Street last year.
Allegra Stratton, his adviser, quit when ITV received video of her joking about a second party in Downing Street.
‘Shaun Bailey AM has stepped down as Chairman of the London Assembly's Police and Crime Committee.
In December, an unapproved social event involving former London mayoral campaign team members distracted from the Committee's critical work holding the Mayor of London accountable.
In addition, he will continue to advocate for a strategy to address the disproportionate level of crime in London's black community.
‘On the evening of December 14th, senior CCHQ personnel were aware of an illicit social event in the basement of [CCHQ] hosted by the Bailey campaign.
‘CCHQ personnel seconded to the Bailey campaign faced formal disciplinary action.'
Daisy Cooper, Lib Dem health spokesman, claimed the snapshot proved ‘just how corrupt Johnson's Conservative Party has become'.
Bailey and his cronies partying at Tory HQ while the British public was shut down and the NHS was saving lives is another punch to the stomach for everyone who followed the rules.
A spate of rule-breaking disclosures reveal Boris Johnson has established a low bar for norms within his party, eroding public trust.'
The Lib Dems have now paid to advertise the Mirror storey exposing the image to voters on Facebook in North Shropshire, where a byelection is scheduled for Thursday.
Cabinet Secretary Simon Case is examining reported gatherings or parties in Government buildings.
He may present his findings as early as this week.
‘Given the ongoing review, it would be improper to comment while that is underway,' said a Government official.