Detectives investigating alleged parties held in Downing Street and Whitehall will begin contacting more than 50 people believed to have taken part in the events by the end of the week, the Met Police said.
In an update, the force said that officers will begin sending formal questionnaires to alleged attendees of eight events that that are being investigated between May 20 2020 and April 16 2021.
It marks a significant step in the police investigation into partygate, which the Met have called Operation Hillman.
The force said the questionnaire would ask for “an account and explanation of the recipient’s participation” in an event which is the subject of police inquiries.
Recipients will be advised that the questionnaire has “formal legal status” and that their response is required within seven days. In most cases the contact will be by email.
Contact will be made with alleged attendees of eight parties - including a “boozy” leaving do held for departing communications chief James Slack held on the evening before Prince Philip’s funeral on April 16, 2021.
Enquiries will also be made about an event allegedly held in Downing Street on November 13, 2020 – the same night that Mr Johnson’s former adviser Dominic Cummings left his post. It is claimed that loud music was heard coming from the flat where Mr Johnson lives with his wife Carrie and their two children.
The Daily Telegraph reported that Mr Johnson was seen heading to the flat that night – leaving open the possibility that the prime minister could be contacted by officers in the coming days.
Mr Johnson has previously said he would be happy to cooperate with the Met Police as they carry out their investigation.
The announcement came after the Met said they were considering widening their inquiries to cover a quiz night in No 10 on December 2020 after a photograph emerged of Boris Johnson and three colleagues near an open bottle of sparkling wine.
Mr Johnson’s premiership has been rocked by the partygate scandal, with a string of Tory MPs withdrawing their support and submitting no confidence letters in recent weeks.
In a bid to ease anxieties among Tory backbenchers, Mr Johnson on Tuesday announced a small reshuffle of his Cabinet, including the promotion of chief whip Mark Spencer to Commons leader and Jacob Rees-Mogg as minister for Brexit opportunities.
It followed the resignation of four of his most senior Downing Street aides last week, several of whom are implicated in the partygate scandal.
Former communications chief Jack Doyle, who resigned last Thursday, reportedly attended at least two of the 12 events in Downing Street.
Martin Reynolds, previously Mr Johnson’s principal private secretary, also announced he would step down from his post after an email was leaked in which he invited Downing Street staff to a “bring your own booze” party held in No10 on May 20, 2020.