In Operation Hillman - the government's probe into Johnson's party on Downing Street, more than 50 people must answer questions about the Downing Street and Whitehall parties, according to Scotland Yard.

The investigation looks into the breach of law regarding COVID-19 restrictions, focusing on 8 dates between May 2020 and April 2021. 

Civil staff and servants, in addition to Boris Johnson and his wife, Carrie, will be sent formal questionnaires with legal status about the events, requesting recipients to testify for the events.

Read more: Johnson's four top aides quit Number 10 amid 'Partygate' fallout

After the recipients send their responses - which must be provided within 7 days - the special inquiries team of the Metropolitan police will be looking into them. If an 'unreasonable' breach of COVID-19 regulations was detected, an FPN - fixed penalty notice - will be filed to the criminal records office, ACRO. 

An image published by The Daily Mirror in December 2020 shows Johnson with his deputy principal private secretary, Stuart Glassborow, with an open bottle of prosecco and a staff member wearing party decorations. Downing Street said that the event was a virtual quiz. 

Conservative members of the parliament, even loyalists, demanded that the PM be removed from his position if investigations reveal he committed a criminal offense. Nevertheless, Johnson is in denial that he has broken any law, calling the suggestion to add the "vitual quiz" to the events "completely in error."

Met police issued a statement: “The Metropolitan Police Service 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." 

A labour MP, Fabian Hamilton, said “It looks a lot like one of the Christmas parties he told us never happened," to which Johnson responded that Hamilton is "completely in error."

At least three of the four events have been attended by Johnson as far as the police know. Johnson refused to disclose whether he was at the fourth event which took place in his apartment on Downing Street.

However, the 15 December quiz was not part of the gatherings which the Met is looking into. 

On Wednesday evening, police issued a statement revealing that they have been handed more than 300 photographs of Downing Street events, in addition to 500 pages of evidence. According to Scotland Yard, more people may be contacted regarding the issue. 

Dominic Cummings, a former governmental advisor, said more pictures will be coming soon: “There’s waaaaay better pics than that floating around, incl[uding] in the flat,” he tweeted against the backdrop of the virtual quiz revelation. “The pics will come out and the public will think ‘Met [police] lying’, not ‘oh PM innocent after all’. Penny dropping with MPs.”

there's waaaaay better pics than that floating around, incl in the flat

— Dominic Cummings (@Dominic2306) February 9, 2022