British Prime Minister Boris Johnson took part in a Christmas quiz sitting between two colleagues in No 10 when indoor mixing was banned in London last year.
The Sunday Mirror published a picture of Mr Johnson at the event, which it said was on 15 December.
Downing Street said the prime minister “briefly took part virtually in a quiz” to thank staff for their hard work during the pandemic.
Labour’s Angela Rayner said Mr Johnson was “unfit to lead this country”.
It comes after the government launched an investigation into three allegations of gatherings last Christmas – including two in Downing Street – despite Covid rules banning them.
In the photo, Mr Johnson is sitting in the No 10 library between a colleague wearing a tinsel scarf and another apparently in a Santa hat.
London was under Tier Two restrictions at the time, which banned mixing of households indoors and allowed a maximum of six people to meet outside.
Official guidance said: “Although there are exemptions for work purposes, you must not have a work Christmas lunch or party, where that is a primarily social activity and is not otherwise permitted by the rules in your tier.”
Responding to the photo, a No10 spokesperson said: “This was a virtual quiz. Downing Street staff were often required to be in the office to work on the pandemic response so those who were in the office for work may have attended virtually from their desks.”
The Department for Work and Pensions has confirmed some of its staff drank alcohol and ate takeaways “late into the evening” on several occasions while Covid restrictions were in place.
It follows a report in the Sunday Mirror alleging that political staff and officials working for minister Therese Coffey frequently drank after work until the early hours of the morning.
On Friday 10 Downing Street cancelled its 2021 Christmas party amid growing anger over a different gathering on 18 December last year, after London had been put under Tier 3 restrictions banning gatherings of two or more people indoors unless it was “reasonably necessary” for work.
After criticism from opposition parties, the media and his own MPs, the prime minister asked Cabinet Secretary Simon Case to carry out an investigation into the 18 December party earlier this week.
The inquiry will also include finding out what happened at another Downing Street gathering on 27 November last year, and another at the Department for Education on 10 December.
Allegra Stratton quit her post as a government spokesperson on Wednesday after a backlash over a video she appeared in from last December, in which she joked about a Christmas party.
The row over Christmas parties comes as the government announced new rules in England to tackle the spread of Omicron.
On Saturday, a further 633 Omicron cases were reported. Overall, a further 54,073 daily cases were recorded across the UK.