Detectives investigating alleged breaches of Covid-19 regulations 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 to get their accounts, the Metropolitan Police have said.

       The inquiry has been named Operation Hillman and will see officers send formal questionnaires to more than 50 people, starting by the end of this week, in relation to eight dates that are being investigated between May 20 2020 and April 16 2021.

       "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," the Met said.

       "Recipients are informed that responses are required within seven days. In most cases contact is being made via email."

       It came after the force said it is reviewing its assessment that a Christmas quiz held in No 10 in December 2020 did not meet the threshold for criminal investigation after an image surfaced of Boris Johnson near a bottle of sparkling wine.

       The photo shows the Prime Minister and two members of staff, one with tinsel draped around their neck, near the bottle of alcohol and an open packet of crisps at the quiz, which happened during Covid restrictions.

       Responding to the photo, the Met Police said: "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 of criminal investigation. That assessment is now being reviewed."

       The event, on Dec 15, 2020, was originally not one of those being investigated by the Metropolitan Police after officers were passed evidence from the Sue Gray inquiry.

       After the photo was published by The Mirror, Dominic Cummings, Mr Johnson's former chief adviser, tweeted: "There's waaaaay better pics than that floating around, incl in the flat.

       Fabian Hamilton, the shadow minister, raised the photograph at Prime Minister's Questions on Wednesday, saying: "It looks a lot like one of the Christmas parties he told us never happened. Will the Prime Minister be referring this party to the police as it is not one of the ones currently being investigated?"

       Mr Johnson responded: "In what he has just said, I'm afraid he is completely in error." Challenged again during PMQs about the photograph, he said: "That event already has been submitted for investigation."

       London was under Tier 2 restrictions – which prohibited social mixing between different households indoors – at the time.

       Official guidance said: "Although there are exemptions for work purposes, you must not have a work Christmas lunch or party where that is a primarily social activity and is not otherwise permitted by the rules in your tier."

       Adam Wagner, a human rights lawyer who has been examining Covid laws, said earlier on Wednesday: "I think there is no longer any justification for the police not to investigate this event.

       "I imagine the reason it was decided not to investigate this gathering is because the image was ambiguous – the PM may have dialled in but not have been himself participating in an illegal gathering. But now it seems obvious from the photo he himself is participating in a social gathering."