Former London mayoral candidate Shaun Bailey has apologised “unreservedly” for attending a party at Conservative headquarters in December last year.

On Tuesday, the Daily Mirror published a photograph showing Tory Mr Bailey – who challenged Sadiq Khan in the capital’s mayoral race – with more than 20 other people wearing party hats or holding drinks.

Next to the group was a table laid with buffet food.

In a tweet thread, Mr Bailey said: “I want to apologise unreservedly for attending a gathering held by some of my staff in my campaign office last December.

“I gave a speech to my team to thank them for their efforts, before leaving shortly afterwards.

“It was a serious error of judgement at a time when Londoners were making immense sacrifices to keep us all safe and I regret it wholeheartedly.”

The Tories previously admitted the party had taken place on December 14, when London was in Tier 2 restrictions and said that staff had been disciplined.

The rules meant that no mixing was allowed indoors apart from with your household or bubble.

Up to six people can meet outside in private gardens or public parks.

The Greater London Authority Conservatives confirmed on Tuesday that Mr Bailey had stepped back from chairing the London Assembly’s police and crime committee.

In a statement, they said: “He does not want an unauthorised social gathering involving some former members of the London mayoral campaign team last December to distract from the committee’s important work holding the Mayor of London to account.

“He will continue to speak up for Londoners who no longer feel safe in our city and push for a strategy to tackle the disproportionate level of crime in London’s black community.”

A Tory spokesman previously said: “Senior CCHQ (Conservative Campaign Headquarters) 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.”