A statement on Wednesday from the Metropolitan Police said officers would start contacting people from the end of this week, asking them to complete a document with formal legal status on the events that have left Johnson facing the gravest crisis of his premiership.
Police are investigating 12 gatherings held at Johnson's office and residence after an internal inquiry found his staff had enjoyed alcohol-fuelled parties in Downing Street, with the British leader attending a few of the events himself.
At the time many people could not attend funerals or say farewell to loved ones dying in hospital due to strict COVID-19 lockdown rules, and the revelations have sparked widespread anger. Some lawmakers in his own party have joined the opposition in calling for him to quit.
Johnson has apologised and promised to change the culture at the top of government after the internal inquiry found a "serious lack of leadership".
After five aides quit, he has appointed new staff to senior roles.
The internal inquiry had found evidence of 16 parties or gatherings, with the police saying they would examine 12 that appeared to meet the threshold for criminal investigation.
They are also now reviewing one of the four other events after a new photograph emerged showing Johnson standing near two members of staff, one of whom is wearing tinsel around his neck, next to an open bottle of champagne.
The revelations around Johnson have raised questions about his judgement after he originally he said there were no parties at Downing Street during the strict lockdowns.
The full internal report by senior official Sue Gray will be published at a later date.
