Rishi Sunak has received a partygate questionnaire from the Metropolitan Police, a Treasury source has confirmed.
The source said it was "inevitable" the chancellor would receive a questionnaire, "given he lives and works in the building".
The Met has sent out the forms to more than 50 people under Operation Hillman, which is looking into 12 reported gatherings.
Sending one to Mr Sunak indicates he is suspected, along with Boris Johnson, of breaking coronavirus regulations.
Earlier this week, ITV News obtained a copy of one of the questionnaires which have been sent out.
Mr Sunak was present during Boris Johnson's birthday party in Cabinet Room on June 19, 2020, however previously claimed to have been there awaiting a Covid-19 meeting.
After ITV News first reported the allegations in January, a spokesperson for Mr Sunak said: "The Chancellor went round to the cabinet room for a Covid strategy committee meeting, which was being held in the same room."
The MP for Richmond in Yorkshire initially told media he did not expect to receive a questionnaire.
Asked on February 11, the chancellor said "no” before pausing and adding: “I don’t know."
Last week ITV News revealed that those under police investigation would be allowed to view the responses they gave in interviews with civil servant Sue Gray before they respond to the police questionnaire.
Ms Gray - who conducted the original internal investigation before passing police evidence which pushed them into announcing their own criminal probe - said people would only be able to view their own answers and no one else's.
Among the package of evidence passed to police by Ms Gray were 300 photographs relevant to the partygate investigation - one of which allegedly shows the PM holding a beer.
The Met has confirmed that in line with policy it would not reveal the people who are issued with FPNs, however Number 10 has said it would be made public if the prime minister is fined.
Fines start at £100 for the first offence, growing to £200 for the second offence before doubling for each repeat offence before hitting the cap of £6,400.
Any individual is entitled to appeal their fine, which could see the appellant appear in court.