The Met Police said it would be sending formal legal questionnaires by the end of the week to more than 50 individuals in No10 regarding illicit gatherings that allegedly broke Covid lockdown rules.
The move means Mr Johnson will have to provide a credible reason as to why he was at events held during coronavirus restrictions or face a fine.
Mr Johnson is facing the most challenging time of his premiership over claims he attended parties while the country was under strict Covid curbs that his government introduced to limit the spread of the virus.
A number of backbench Tory MPs have already called for him to resign and on Thursday former prime minister Sir John Major used a speech to accuse Mr Johnson and No 10 officials of breaking lockdown laws and concocting “brazen excuses” about the partygate affair.
The Met is currently investigating 12 events as part of Operation Hillman and are sending the questionnaires to more than 50 individuals across Downing Street and wider government.
The force say the questionnaires ask for an “account and explanation of the recipient’s participation in an event” and have “formal legal status and must be answered truthfully”.
A spokesman previously told The Independent that answers provided “will be treated as written statements made under caution”. Police cautions are used when an individual is suspected of a criminal offence.
The prime minister is alleged to have been at up to six gatherings, including the “bring your own booze” party in the No 10 garden in May 2020 during the first lockdown.
Mr Johnson allegedly attended another organised by his wife Carrie Johnson in No10 in November that year, during which Abba songs were reportedly heard on the night of former chief adviser Dominic Cumming’s departure.
On Friday evening his former aide Mr 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.”
A No 10 spokesperson said: “We can confirm the Prime Minister has received a questionnaire from the Metropolitan Police. He will respond as required.”
The Met Police declined to comment further when approached on Friday evening.