Chancellor Rishi Sunak is facing questions from police investigating allegations of lockdown-breaching parties in Whitehall.
Sources close to the chancellor said he had received a questionnaire from the Metropolitan Police asking him to confirm whether he attended the events under investigation and what excuse he had for being there.
The move means that Mr Sunak is effectively being questioned under caution, and could face a fixed penalty fine of £100 or more for breaching Covid regulations.
Mr Sunak was present at an impromptu birthday party arranged for Mr Johnson in No 10’s cabinet room on 19 June 2020.
Police are understood to have been passed a picture of the event taken by an official photographer, which reportedly shows Mr Johnson holding a can of beer and Mr Sunak with a soft drink.
A Treasury source has previously said that the chancellor joined the party inadvertently after going to the cabinet room for a Covid strategy committee meeting.
Around 30 people were present at the event, where cake was served, at a time when indoor gatherings were banned under strict coronavirus legislation.
Questionnaires were sent by the Met’s Operation Hillman to 88 people including the prime minister earlier this month, asking them to provide a “lawful exception” or “reasonable excuse” for their presence at any of the 12 events under investigation.
The document states at the outset that those accused have an opportunity to provide “a written statement under caution”.
It then goes on to ask around a dozen questions, including timings of attendance at parties and how many others were present.
It also asks respondents if they “interacted with” or undertook “any activity with” any one present.
The form gives them a chance to justify their actions, asking: “What, if any, lawful exception applied to the gathering and/or what reasonable excuse did you have for participating in the gathering?”