The email to alleged partygoers must be answered truthfully and will hold legal status, the Met says.
The Metropolitan Police will email more than 50 people as part of its inquiry into lockdown parties at Downing Street and Whitehall.
A questionnaire will be sent to people alleged to have been at any of eight events between May 2020 and April 2021.
It will ask for their account of what happened and “must be answered truthfully,” the Met said.
The document will hold legal status and must be answered in seven days, the force said.
The Met has called its investigation into alleged breaches of Covid-19 regulations in government buildings Operation Hillman.
It earlier announced it would review its decision not to investigate a Christmas quiz in No 10 after a picture of Boris Johnson was leaked to the press.
The photo, published by the Daily Mirror, showed the PM standing next to a bottle of bubbly alongside staff.
The newspaper said the photo was taken on 15 December 2020 – while Covid restrictions were in place – during a No 10 “virtual” Christmas quiz.
But it is not one of the parties currently under police investigation.
A statement from police said: “The [Met] 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.”
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