Boris Johnson's press chief Jack Doyle spoke at No 10 party last year

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 communications director, gave a speech to between 20 and 30 people at the 18 December event.

A source has told the BBC there were food, drinks and games at the event.

Labour leader Sir Keir Starmer said the prime minister was "not fit for office" and it was important to "get to the bottom of this".

But Downing Street said: "There is an ongoing review, and we won't be commenting further while that is the case."

This 18 December event is one of three government staff gatherings from last year now being investigated by the UK's top civil servant, Simon Case, for possible breaches of Covid rules.

The others are a No 10 staff event on 27 November last year and a gathering at the Department of Education event on 10 December. The inquiry could be widened out if it considered there are credible allegations about other events.

Foreign Secretary Liz Truss has said she did not hold a Christmas party last year, when she was international trade secretary, as she had been too "busy".

"We had a lot of deals to get done by 31 December, so that was my focus," she told the BBC's Political Thinking with Nick Robinson podcast.

The revelation of Mr Doyle's speech at the Downing Street gathering follows an arduous few days for the government, during which time it has been accused by Labour of not being honest over this and other events.

ITV News, which first reported that Mr Doyle had been present at the 18 December party, said he had also handed out award certificates to staff.

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 the restrictions.

On Wednesday, the prime minister told the Commons he had been "repeatedly assured" that there had been no party and that no Covid rules had been broken on that date.

The government's chief whip Mark Spencer told BBC Radio Nottingham that Mr Johnson "will not have lied" about any parties that took place on Downing Street last year.

He added that the 18 December event had been a "meeting" rather than a social get-together, and that staff had discussed how to deal with coronavirus.

But Sir Keir, speaking during a visit to Northumberland, said: "It's absolutely important that we get to the bottom of this.

"This is just the latest allegation of dishonesty from the prime minister. We've had lie upon lie in relation to the parties going on in Downing Street. The prime minister is not fit for office."

Government adviser Allegra Stratton quit her post on Wednesday after a backlash over a video she appeared in from last December, in which she joked about a Christmas party.

Before her resignation, Mr Johnson told MPs that he was "furious" about the clip, obtained by ITV News, and was launching an investigation into whether Covid rules had been broken.

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?

Read Laura's blog in full

Meanwhile, Lord Geidt, the prime minister's standards adviser, is unhappy about the contents of an Electoral Commission report into the funding of refurbishments to the Downing Street flat but is not ready to resign over it, the BBC understands.

Labour says the report shows Mr Johnson lied to Lord Geidt when he investigated the PM's conduct earlier this year, something denied by Downing Street. Labour has written to Lord Geidt, asking him to reinvestigate.

Mr Johnson is also facing a rebellion from Conservative MPs over his plans announced on Wednesday to introduce tighter Covid curbs in England to tackle the Omicron variant.

Dozens of Tories have hit out at extra restrictions ahead of a vote next week, particularly the plans for mandatory Covid passes for nightclubs and large events.

Former cabinet minister Andrew Mitchell said the mood in the Conservative Party was "sulphurous", adding: "What we need now is a bit of grip from No 10."

Business minister Paul Scully told BBC Breakfast it had been a "difficult week for the government" and that it had not "been able to get across" policies it had announced to deal with Covid and the economy.