The force said it had now begun interviewing witnesses and may need to contact more over the gatherings.

The Met launched its investigation in January after a string of reports about lockdown parties across Whitehall.

It is formally investigating 12 events, including three the PM attended.

Boris Johnson has refused to say whether he was at another alleged event, said to have been held in his Downing Street flat in November 2020.

Leader of the Liberal Democrats, Sir Ed Davey, called on the prime minister "come clean" and reveal if he was being interviewed by the police, as well as whether he would resign if found to have broken the law.

He added: "No more fluff, just the facts for all those who suffered during lockdown while he partied."

Initial findings from an inquiry by senior civil servant Sue Gray said there had been "failures of leadership and judgement" over the parties, as well as excessive drinking at work against the backdrop of the pandemic.

She has pledged to update her report after the police finish their investigation.

Questions and answers

In a statement on Monday, the Met said detectives from the Operation Hillman investigation team had started interviewing "key witnesses".

The force added: "To date, over 100 questionnaires have been sent out asking the recipients about their participation in alleged gatherings.

"The questionnaires provide recipients with the opportunity to give an account of their involvement and the responses form part of the investigative material for the operation."

All recipients - including both the prime minister and Chancellor Rishi Sunak - were given seven days to respond.

The statement said: "As a result of responses so far, further individuals have been identified and questionnaires sent to them.

"As the investigation continues, we may need to contact more people as further information comes to light."

The force has not issued any fixed penalty notices for breaching of lockdown rules in connection with events in Downing Street and Whitehall.

But the Met added: "Every questionnaire response is being assessed alongside all available evidence, and should this reach the evidential threshold, then referrals will be made."

The Met's statement seems to have dashed any hopes that No 10 may have had for a swift conclusion to the so-called "partygate" probe.

Indeed, inquiries into events in Downing Street and nearby government buildings in 2020 and 2021 appear to be expanding.

And there have been suggestions that the size of the police inquiry team is to be increased to deal with the volume of work.

I was told that some Downing Street staff had felt aggrieved to have received police questionnaires, while some of their colleagues who had attended the same events had not.

It now seems that some of them may have been pointing fingers, leading to more police questionnaires being issued and elongating the whole process.

And the full report by senior civil servant Sue Gray can't be published until the police conclude their inquiries.

Two Conservative MPs have withdrawn letters of no confidence in the prime minister, believing it would be wrong to hold a leadership contest against the backdrop of the Ukraine conflict.

But some others have told me they have no intention of doing so, and that a long-running investigation has the potential to inflict serious political damage to Boris Johnson.

Sue Gray’s record of the gatherings

The government has faced intense pressure over gatherings held in and around Downing Street during Covid lockdowns. Senior civil servant Sue Gray has said that many of them “should not have been allowed to take place or to develop in the way that they did.” Here is what we know about them and the restrictions in place at the time:

15 May 2020

A photo from May 2020 showed the prime minister and his staff with bottles of wine and a cheeseboard in the Downing Street garden. When asked about it, Boris Johnson said “those people were at work talking about work”.

The rules:

Legal restrictions at the time said you could not leave your house without a reasonable excuse and government guidance was that you could meet one person outside of your household in an outdoor setting while exercising.

This event is not being investigated by the police.

20 May 2020

About 100 people were invited by email to “socially distanced drinks in the No 10 garden this evening”. Witnesses told the BBC the PM and his wife were among about 30 people who attended. Boris Johnson has declined to say whether he was among those there.

This event is being investigated by the police.

18 June 2020

A gathering took place in the Cabinet Office to mark the departure of a No 10 private secretary.

This event is being investigated by the police.

19 June 2020

On Boris Johnson’s birthday, up to 30 people gathered in the Cabinet Room at No 10 to present the prime minister with a birthday cake and sing Happy Birthday, according to a report by ITV News.

No 10 said staff had “gathered briefly" to "wish the prime minister a happy birthday", adding that he had been there "for less than 10 minutes”.

The rules:

Restrictions at the time banned most indoor gatherings involving more than two people.

This event is being investigated by the police.

13 November 2020

Sources told the BBC that Downing Street staff members attended a gathering with Carrie Johnson in the flat where she and the prime minister live. A spokesman for Mrs Johnson denies the party took place. There was a separate gathering in No 10 Downing Street on the same day to mark the departure of a special adviser.

The rules:

Eight days earlier Boris Johnson had announced a new lockdown in England. Indoor gatherings with other households were banned, unless they were for work purposes.

These events are being investigated by the police.

27 November 2020

A leaving event was held for No 10 aide, Cleo Watson, where people were drinking, and Mr Johnson made a speech, according to sources.

This event is not being investigated by the police.

10 December 2020

The Department for Education has confirmed it had an office gathering to thank staff for their work during the pandemic. It says drinks and snacks were brought by those who attended and no outside guests or support staff were invited.

The rules:

Eight days earlier, London had been placed in restrictions which banned two or more people from different households from meeting indoors, unless “reasonably necessary” for work purposes.

This event is not being investigated by the police.

14 December 2020

The Conservative Party has admitted that an “unauthorised gathering” took place at its HQ in Westminster. It was held by the team of the party's London-mayoral candidate, Shaun Bailey, who has since stepped down as chair of the London Assembly police and crime committee. The Metropolitan Police is to speak to two people who attended the party.

15 December 2020

Multiple sources have told the BBC there was a Christmas quiz for No 10 staff last year. A photo - published by the Sunday Mirror - shows Boris Johnson taking part and sitting between two colleagues in No 10. Mr Johnson has denied any wrongdoing.

This event is not being investigated by the police.

16 December 2020

The Department for Transport has apologised after confirming reports of a party in its offices, calling it “inappropriate" and an "error of judgment” by staff.

The rules:

London moved into the highest tier of restrictions and Matt Hancock, who was health secretary at the time, said it was important “everyone is cautious” ahead of the festive period.

This event was not included in Sue Gray's report.

17 December 2020

A leaving party was held at the Cabinet Office for the outgoing head of the civil service Covid taskforce - the team responsible for drawing up coronavirus restrictions.

Kate Josephs, now chief executive of Sheffield City Council, apologised for the event, saying she was “truly sorry that I did this and for the anger that people will feel as a result”.

A second gathering was held in the Cabinet Office to hold an online Christmas quiz for the Cabinet Secretary’s private office.

A third gathering was held in No 10 Downing Street to mark the departure of a No 10 official

These events are being investigated by the police.

18 December 2020

Downing Street originally denied a report by the Daily Mirror that a party took place in Downing Street. However, a video obtained by ITV News showed the prime minister's then-press secretary Allegra Stratton, joking about reports of an event, saying: “This fictional party was a business meeting and it was not socially distanced.”

This event is being investigated by the police.

14 January 2021

A gathering was held in No 10 Downing Street to mark the departure of two private secretaries.

This event is being investigated by the police.

16 April 2021

Two parties were held by Downing Street staff at No 10, the night before Prince Philip's funeral.

One of the events was a leaving party for the PM's then director of communications James Slack, who has apologised for the event and acknowledged it “should not have happened at the time that it did”.

Boris Johnson was not at either party.

The rules

The rules had been eased in England on 12 April, but working from home continued to be recommended and socialising indoors with people from other households was not allowed. Meeting others outdoors was limited to groups of six people or two households.

These events are being investigated by the police.