Boris Johnson has responded to a questionnaire from the Metropolitan Police about Downing Street gatherings during coronavirus restrictions.
A Number 10 spokesperson confirmed to Sky News that the prime minister has returned the document.
Downing Street has previously said Mr Johnson’s responses will not be made public.
The PM had seven days to respond to the questionnaire, which was sent to him last Friday.
Questionnaire has legal status
The document, which has been sent to all those believed to have attended alleged COVID-19 lockdown-busting parties, asks for the recipient’s account and an explanation of their participation in an event.
Scotland Yard has stated the questionnaire has formal legal status and must be answered truthfully.
The force has been contacting more than 50 people believed to have been involved in the 12 events it is investigating as part of Operation Hillman, which was launched in the wake of the findings of a Cabinet Office inquiry into the partygate allegations.
The Met has said those contacted will not necessarily be given a fine.
But a statement earlier this month said: “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.”
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The Association of First Division Civil Servants (FDA), a union which represents civil servants, revealed it had called for officials involved in the investigation to be able to consult notes on the evidence they gave to the Cabinet Office inquiry to help inform their Met Q&A.
According to ITV News, staff were told they can look at notes on their own interview in a letter from Sue Gray, the senior civil servant who led the inquiry.
The Cabinet Office has declined to comment.
How did we get here?
Allegations of lockdown-busting events in Downing Street and across Whitehall in 2020 and 2021 have piled pressure on the PM, with some of his own backbench MPs joining opposition parties in calling on Mr Johnson to go over the row.
A partial version of the Cabinet Office inquiry said Downing Street lockdown gatherings represented a “serious failure” and were “difficult to justify”.
The PM apologised in the wake of the highly-critical report and promised changes at the top of his Number 10 operation, with a number of senior aides departing as part of a Downing Street shake-up.
After the release of the initial Gray report, which looked at 16 gatherings, the Met announced it would be investigating a dozen of those events for potential breaches of COVID regulations.
These include an event that happened at the Downing Street flat on 13 November 2020, the night aides Dominic Cummings and Lee Cain left their Number 10 roles, and an event to mark the PM’s birthday on 19 June 2020.
The PM is reported to have attended as many as six of the 12 events under investigation.
Last week, a fresh photo emerged of Mr Johnson at a Christmas quiz in Number 10, an event that police initially decided not to investigate.
But the Met is now rethinking its assessment of that event following the publication of the image of the PM next to an opened bottle of alcohol, crisps, a person wearing tinsel and another wearing a Santa hat.
Ms Gray has handed more than 300 images and more than 500 pages of information to Scotland Yard.
How much trouble is the PM in?
A total of 14 Tory MPs have so far called for Mr Johnson to resign, but the momentum behind the internal party pressure for the PM to go has waned in recent weeks.
Many MPs are thought to be waiting for the outcome of the police investigation into partygate.
If Mr Johnson is fined, this would likely provoke a fresh wave of calls for him to go.
The PM has so far dismissed the notion of resigning over the row, meaning Conservative critics will have to force a no-confidence vote in his leadership of the party.
Read more: Which Conservative MPs have called on the prime minister to quit?
A total of 54 letters – from 15% of the party’s 360 MPs – are needed to trigger a leadership vote.
If Mr Johnson were to win such a ballot, he would be immune from another leadership challenge for a year.
Were he to lose, he would have to resign and a successor as party leader and PM would be chosen.