Pressure on the government is growing following a wave of anger over allegations various Covid rule-breaking events were held in No10 while the capital was in Tier 2 and 3 last year.
A Tory spokesperson confirmed the “unauthorised” event took place, while the Times newspaper claims Mr Bailey attended the party himself, in the evening of December 14.
At the time London was in Tier 2 restrictions, where Londoners could only meet people from other households outside, and the rule of six applied.
That day then-health secretary Matt Hancock announced the capital would be moving to Tier 3, with pubs and restaurants ordered to shut, due to a rise in Covid cases.
Now Labour assembly member Leonie Cooper has urged Mr Bailey to step down from his role as chairman of the assembly’s police and crime committee, which scrutinises Mr Khan and Met commissioner Dame Cressida Dick’s work to lead the force and keep the city safe.
In a letter posted on Twitter, Ms Cooper, Labour group deputy leader at City Hall, wrote: “Did you or did you not attend the party on 14th December? Who else was in attendance?
“Whilst the details of this event are established and whilst any necessary investigations are underway, I believe your continued chairmanship of the police and crime committee is untenable.
“I am asking that on this occasion you do the right thing and step down as chair, with immediate effect until the necessary enquiries have seen completion.”
Speaking to LondonWorld, Ms Cooper, member for Merton and Wandsworth, said: “Everyone is actually really angry.
“To see that potentially also the mayoral candidate was possibly also attending a party... I think people are furious.
“I thought I should write to him because we should be told whether he was there.
“It seems unlikely a party would be thrown for the candidate and the staff without the candidate being there, but I think Shaun needs to tell us.
“It’s a bit difficult for him to stay on as the chairman of the PCC. Some of that could potentially end up under investigation if the police choose to do that.”
According to reports, No10 aides were also at the gathering, which saw party-goers drink wine and dance into the early hours, despite mixing socially indoors then being forbidden.
A Conservative Party spokesperson said: “Senior Conservative Campaign Headquarters (CCHQ) staff became aware of an unauthorised social gathering in the basement of Matthew Parker Street organised by the Bailey campaign on the evening of December 14.
“Formal disciplinary action was taken against the four CCHQ staff who were seconded to the Bailey campaign.”
The Met Police have been approached for comment.
Shaun Bailey has been approached for comment.