Boris Johnson has received his questionnaire from police investigating allegations of lockdown-breaching parties in No 10, Downing Street has said.
On Wednesday, detectives investigating partygate said they would begin contacting more than 50 attendees this week.
The Metropolitan Police said it would be sending notices asking for “an account and explanation of the recipient’s participation in an event”.
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 Johnson will have to provide a credible reason as to why he was at events during coronavirus restrictions or face a fine for breaking coronavirus laws.
Officers working on so-called Operation Hillman are investigating 12 events.
The prime minister is alleged to have been at up to six of them, including the “bring your own booze” party in the No 10 garden in May 2020 during the first lockdown.
Johnson allegedly attended another organised by his wife Carrie Johnson in the official Downing Street residence in November that year, during which Abba songs were reportedly heard on the night of former chief adviser Dominic Cumming’s departure.
Cummings tweeted: “great work tory mps leaving this crippled joke of a pm spending next 7 days bunkered down with lawyers trying to remember all his different lies while another major global crisis unfolds.”
The development puts the embattled PM’s future in greater jeopardy.
Fifteen Tory MPs have publicly called for him to go, while more are thought to have privately written to the 1922 Committee of backbench Tories calling for a no confidence vote.
More still are poised to do so if Johnson is found to have broken his own coronavirus laws, or further damaging details emerge from the Sue Gray inquiry.
He will face a vote of no confidence if 54 Conservatives write to 1922 Committee chair Sir Graham Brady, and would be ousted if more than half of his MPs subsequently voted against Mr Johnson.
The questionnaire was also dispatched during a difficult time for the Met, with commissioner Cressida Dick having been pressured into announcing her resignation after a series of scandals.