A picture published in the Sunday Mirror showed the PM at a virtual quiz in Downing Street, appearing on screen alongside two colleagues.

But sources have told the BBC other No 10 staff members sat in groups of six in a room in Cabinet Office.

Mr Johnson denied any wrongdoing but said the event would be investigated.

The prime minister's spokesman confirmed senior civil servant Simon Case, who was tasked with the probe into government parties last week, was now investigating the quiz.

It means he is looking into four events in total.

The PM said Mr Case would report back "as soon as we reasonably can".

The government has been under increasing pressure over lockdown gatherings after the news broke of a party in Downing Street on 18 December 2020 - when London was under Tier 3 restrictions and the announcement of a new country-wide lockdown was looming.

Mr Johnson said he had been assured all rules were followed, but a leaked video of No 10 staffers joking about the event led to one of them resigning and the PM asking the Cabinet Office to launch an investigation.

Other allegations about parties across government departments began to emerge, including several sources telling the BBC about a quiz on 15 December, organised in advance by No 10 staff, with groups of employees gathered in a policy unit room to take part.

On Sunday, a picture of Mr Johnson taking part in the event - when the capital was under Tier 2 restrictions - was published by the Sunday Mirror, which claimed he hosted a round of the quiz alongside colleagues, who were wearing tinsel and a Santa hat.

The picture published by the Sunday Mirror showed Boris Johnson in the No 10 library under a picture of Margaret Thatcher

A Downing Street spokesman said the PM "briefly took part virtually" to thank staff for their work during the pandemic.

But Labour leader Sir Keir Starmer said it appeared Mr Johnson had breached Covid laws in place at the time, telling the BBC's Andrew Marr Show the event had created "fury" and that trust in the prime minister was at an "all-time low".

Asked about the allegations on Monday, the prime minister told reporters: "I can tell you once again that I certainly broke no rules. All that is being looked into.

"Of course all that must be properly got into and you will be hearing from the cabinet secretary about it all."

When asked to confirm Mr Case's investigation would look into his actions, Mr Johnson said: "He's looking at all these things.

"We will report back on all that. I certainly broke no rules and [we will report back] as soon as we reasonably can."

A spokesman for the Cabinet Office said they would not be commenting further, but the terms of reference for the investigation said: "Where there are credible allegations relating to other gatherings, these may be investigated."

What parties took place and when?

13 November 2020: Sources have told the BBC there were impromptu drinks to mark the exit of the former director of communications at No 10, Lee Cain - but it was staff having drinks at their desks and was over by 20:30 GMT.

13 November 2020: Also the date Dominic Cummings left as chief adviser to the PM. Sources told the BBC that several Downing Street staff members attended a gathering with Carrie Johnson in the flat where the prime minister and his wife live above No 11 after exits. A source said music was blaring and could be heard elsewhere in the building. But others said to be at the party denied it took place, as did a spokeswoman for Mrs Johnson.

25 November 2020: The Times reported Treasury officials held a drinks party in their office to celebrate Rishi Sunak's spending review during the lockdown. The newspaper said two dozen civil servants were understood to have attended the event. But a spokesman for the department insisted it was an "impromptu" event, with a "small number" of staff who celebrated around their desks.

27 November 2020: Another No 10 aide, Cleo Watson, left Downing Street marked by an event. Again, sources said it was not formally organised, but people were having drinks and Mr Johnson made a speech.

December 10 2020: The Department for Education has confirmed it had a gathering in the office to thank staff for their work during the pandemic. Drinks and snacks were brought by those who attended and no outside guests or support staff were invited.

14 December 2020: The Conservatives have confirmed a report in the Times that there was an "unauthorised social gathering" in the basement of their party's building, held by the team of the London mayoral candidate at the time, Shaun Bailey. It was described as "raucous", with No 10 aides

15 December 2020: Multiple sources told the BBC there was a Christmas quiz for No 10 staff in the Cabinet Office, with invites sent out in advance via email, telling people to form teams of six. Downing Street have said it was a "virtual gathering" but while some joined in that way, sources said there were groups in the room sat in their teams. This is the event the Sunday Mirror released a picture of the PM attending virtually.

18 December 2020: A Downing Street Christmas party took place on this date, as first reported by the Mirror. A source told the BBC there was food, drinks and games at the gathering that went on past midnight.

DWP gatherings: The Department for Work and Pensions confirmed a "core team" of staff drank alcohol and ate takeaways "on a number of occasions" whilst working late in the office while Covid restrictions were in place. It followed a report in the Sunday Mirror alleging political staff and officials working for minister Therese Coffey frequently drank after work until the early hours of the morning.