Downing Street’s press chief addressed No 10 staff at a 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 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 is one of three government staff events currently being investigated by the UK’s top civil servant, Simon Case.
Mr Doyle, now Downing Street’s director of communications, is understood to have also given out awards to the No 10 press team at the gathering.
The BBC has been told he was also attending Covid meetings in No 10 that night, which went on until late in the evening.
It is understood that every Friday Mr Doyle would thank staff for working hard and give out awards.
ITV News, who first reported Mr Doyle was present at this event, said that on this occasion on 18 December he handed out certificates to staff.
No 10 has refused to explain how the gathering complied with Covid regulations in force at the time, despite a week of scrutiny.
On Wednesday, the prime minister said that he had been “repeatedly assured” that there had been no party and that no Covid rules had been broken on that date.
The BBC’s Political Editor Laura Kuenssberg said Mr Doyle’s presence at the event mattered because, as the current director of communications, he had been in charge of the strategy for the past eight days to say that no party had taken place.
Cabinet Secretary Simon Case, who is carrying out the investigation, is also looking into a No 10 staff event on 27 November last year, and a gathering at the education department on 10 December.