Boris Johnson has tonight received a legally-binding questionnaire from police investigating allegations of lockdown-breaching parties inside No 10.
A spokesman said the Prime Minister would 'respond as required'.
The development comes after a Covid laws expert said the PM could face a £10,000 fine over 'partygate' - if Scotland Yard finds regulations were broken - and the resignation of Metropolitan Police Commissioner Cressida Dick.
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The Met confirmed 'formal questionnaires' would be sent out by the end of this week asking people believed to have taken part in 12 events at Downing Street to provide 'an account and explanation' of what took place.
The PM is alleged to have been at up to six of them.
The force is also reviewing its decision not to investigate a Downing Street Christmas quiz in December 2020.
Formal questionnaires are being sent to more than 50 people in total under Operation Hillman - the police investigation into the allegations.
The force said: "This document, which asks for an account and explanation of the recipient's participation in an event, has formal legal status and must be answered truthfully.
"Recipients are informed that responses are required within seven days.
"In most cases contact is being made via email."
A No 10 spokesperson said on Friday night: "We can confirm the Prime Minister has received a questionnaire from the Metropolitan Police.
"He will respond as required."
The move means Mr Johnson will have to provide a credible reason as to why he was at events held during coronavirus restrictions.
The Met added: "Detectives, working for the Met's Special Enquiry Team, continue to examine more than 500 documents and 300 images provided to them by the Cabinet Office and will be requesting further information from the Cabinet Office to assist with the investigation.
"There may be also be a need to contact further people in the coming days and weeks - should they be identified as having taken part in an event and having potentially breached regulations.
"It should be noted that being contacted does not mean a fixed penalty notice will necessarily be issued to that person.
"Nevertheless, if following an investigation, officers believe it is appropriate because the Covid regulations have been breached without a reasonable excuse, a fixed penalty notice will normally be issued.
"We understand the interest in and impact of this case, and are progressing the investigation at pace. We are committed to completing our investigations proportionately, fairly and impartially."
Meanwhile, the Chancellor appeared to suggest that he does not expect to be asked to fill out a police questionnaire about his involvement in an event that possibly broke coronavirus lockdown rules.
Rishi Sunak has confirmed he was at the surprise birthday celebration for the Prime Minister on June 19 2020 in the Cabinet room in No 10, but has said he was only there for a scheduled Government meeting.
Mr Sunak, asked by Sky News whether he had received a questionnaire from the Metropolitan Police, said: "No."
Asked whether he expected to receive a form to complete, he replied: "No… well, I don't know."
He also said he did not think he had broken Covid rules.
Pressed on whether he has confidence in Boris Johnson still, Mr Sunak added: "The Prime Minister has my total support."
