Cops will quiz more than 50 people who are believed to have been at rule-busting Downing Street parties, the Met has confirmed.
Cressida Dick's force has officially named the probe Operation Hillman and will be sending "formal questionnaires" in the coming days asking party-goers for "an account and explanation".
The Met has warned the forms have "legal status and must be answered truthfully".
It comes as embattled Boris Johnson tries to avoid Tory MPs triggering a confidence vote and ousting him as Prime Minister.
A statement by the Met Police said: "Detectives investigating allegations of breaches of Covid-19 regulations in Downing Street and Whitehall will by the end of this week begin contacting people believed to have taken part in the events in question to get their accounts.
"The officers, from what has been named Operation Hillman, will be sending formal questionnaires to more than 50 people.
"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."
The force adds there may be "a need to contact further people in the coming days and weeks".
The statement said: "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."
Scotland Yard has already said any punishments are likely to be limited to a fixed penalty notice and it will not name anyone it fines.
However, No10 has promised it will confirm if Boris Johnson receives a fine.
Labour's Deputy Leader Angela Rayner tweeted: "The Prime Minister was at the majority of these parties.
"Boris Johnson under investigation by the police and he still can't do the right thing and resign. Enough is enough."
It comes hours after Scotland Yard hinted it could add a 13th party to its investigation into alleged lockdown breaches.
A 'virtual' quiz on 15 December 2020, hosted by Boris Johnson, was one of four events that Scotland Yard looked at without launching a full investigation.
But today the Mirror revealed an image of the Prime Minister flanked by three members of staff, one wearing tinsel and another a Santa hat, and an open bottle of bubbly at a “virtual” Christmas quiz on December 15, 2020.
In response to our revelations, Scotland Yard said it would consider whether to add the quiz to the 12 gatherings it is already investigating in Downing Street and Whitehall.
The Metropolitan Police said in a statement: “The MPS previously assessed this event and determined that on the basis of the evidence available at that time, it did not meet the threshold for criminal investigation.
"That assessment is now being reviewed.”
According to The Times, the PM's allies claim he will not resign - even if he is fined by the Met for breaching Covid rules.
But one Tory MP elected in 2019 told the Mirror: “He will be gone this year, it's just a question of the tipping point.”
A senior Tory added it would be "terminal" if the Prime Minister was fined by Scotland Yard or found to have lied to Parliament.
A third Conservative suggested it was only a matter of time before Boris Johnson is ousted, saying: “The trend is clear - it’s just a matter of the timetable.”
They added: “People needed something to hang their hat on. The Jimmy Savile comments were that thing for some people - the police investigation could be too.”
