The PM’s press chief addressed staff and gave out awards at a Downing Street party last Christmas that is now under investigation, it is understood.
Jack Doyle, then deputy director of communications, gave a speech to 20-30 people at the gathering on 18 December.
A source has told the BBC there were food, drinks and games at the event.
Downing Street said: “There is an ongoing review, and we won’t be commenting further while that is the case.”
This event is one of three government staff gatherings from last year now being investigated by the UK’s top civil servant, Simon Case.
Labour’s deputy leader Angela Rayner said news of Mr Doyle’s attendance had “exposed” Mr Case’s inquiry as a “sham”.
Mr Doyle was also attending Covid meetings in No 10 that night, which went on until late in the evening, the BBC has been told.
It is understood every Friday Mr Doyle would thank staff for working hard and give out an award.
ITV News, which first reported that Mr Doyle was present at this event on 18 December, said he had also handed out award certificates to staff on this occasion.
The event took place two days after London went into Tier 3 lockdown restrictions, meaning people were told not to mix indoors with anyone outside their household or support bubble.
No 10 has refused to explain how the party complied with Covid restrictions in force at the time, despite a deepening row and days of questioning by reporters.
Sources in Westminster are questioning whether it’s possible for Jack Doyle to stay in his job. In part that’s because one of the problems this week for No 10 has been their efforts to deny and explain what did or didn’t happen.
Ministers, and the prime minister himself, have been stuck in the Kafka-esque position of saying that they are sure no rules were broken, but they also don’t know what went on.
If they don’t know what happened, how can they be sure that nothing went wrong?
And if multiple sources have said there was a gathering of several dozen people, and people who were not on the Downing Street payroll had been invited, how can that have been just a few work drinks at the desk in a Covid-secure office?
Mr Doyle, well-liked by his colleagues, is – as director of communications – in charge of the government’s messaging. This week the message has misfired, which makes his confirmed attendance a very big problem.
And for government spinners over the years have often found, there is one fundamental error they cannot make. That’s to become part of the story themselves, an uncomfortable position that Boris Johnson’s press chief now finds himself in.