The Metropolitan Police is investigating at least 12 gatherings, including a "gathering in the No 10 Downing Street flat" on 13 November, 2020 - the night Mr Johnson's former aides Dominic Cummings and Lee Cain left their roles.

Also under investigation is the 19 June 2020 event in which Mr Johnson was "ambushed by cake", in the words of minister Conor Burns, in the Cabinet Room at No 10 to mark the Prime Minister's 56th birthday. Mr Burns later insisted Mr Johnson had told him there was no cake.

Boris Johnson's birthday

Carrie Johnson, the Prime Minister's wife, reportedly organised the surprise complete with a chorus of "happy birthday" and interior designer Lulu Lytle also admitted attending while carrying out controversial work on their Downing Street flat.

In a statement, the Metropolitan Police said: "Our position from the start has been that, while we don't normally investigate breaches reported long after they are said to have taken place, if significant evidence became available we would assess it. That is now the situation and why we have acted.

"As part of the investigation it is necessary for us to contact those who attended these events to get their account. As a result, the Met has requested that any information identified as part of the Cabinet Office investigation about these events, is not disclosed in detail. This request only applies for the duration of our investigation and does not apply to events we are not investigating."

The force added: "Having received the documentation from the Cabinet Office on Friday 28 January, we are now reviewing it at pace to confirm which individuals will need to be contacted for their account. This prioritisation will include reviewing all the material from the Cabinet Office, which includes more than 300 images and over 500 pages of information.

"If following an investigation, officers believe it is appropriate, because the Covid regulations have been breached without a reasonable excuse, a fixed penalty notice would normally be issued. Once the penalty is paid, the matter is considered closed. Alternatively individuals may decide to dispute the notice. In these circumstances officers will consider whether to pursue the matter in a magistrates' court."

PM's apology

Mr Johnson apologised to the Commons after the publication of a report by senior civil servant Sue Gray and promised a shake-up of No 10.

He said: "I get it and I will fix it."

Downing Street said an updated report would be published following the conclusion of the police investigation.

It came after Mr Johnson repeatedly refused to guarantee a fuller version of Ms Gray's work would be published.

A No 10 spokesman said: "Given the police have said they are investigating a number of events, it would not be appropriate to comment further while the Met's investigation is ongoing.

"But, at the end of the process, the Prime Minister will ask Sue Gray to update her work in light of what is found. He will publish that update.

"However the Prime Minister is clear we must not judge an ongoing investigation and his focus now is on addressing the general findings."

Sue Gray's report

Ms Gray's report found there were a series of "failures of leadership and judgment" within No 10 and the Cabinet Office while England had been placed under coronavirus restrictions in 2020 and 2021.

However, she was unable to provide meaningful findings because of the police investigation into at least 12 gatherings linked to government properties in Downing Street and Whitehall.

Ms Gray's report listed 16 events she had examined as part of her inquiry, but said only four were not now being investigated by the police.

"Unfortunately, this necessarily means that I am extremely limited in what I can say about those events and it is not possible at present to provide a meaningful report setting out and analysing the extensive factual information I have been able to gather," she said.

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'Failures of leadership and judgment'

Her report about the wider culture within the "heart of Government" said: "Against the backdrop of the pandemic, when the Government was asking citizens to accept far-reaching restrictions on their lives, some of the behaviour surrounding these gatherings is difficult to justify," she said.

"At least some of the gatherings in question represent a serious failure to observe not just the high standards expected of those working at the heart of Government but also of the standards expected of the entire British population at the time."

She said there was "too little thought" given to what was happening in the country at the time and "failures of leadership and judgment by different parts of No 10 and the Cabinet Office at different times".

"Some of the events should not have been allowed to take place," she said.

"Other events should not have been allowed to develop as they did."