The National Audit Office (NAO) is examining the decision to use £220,000 from taxpayers' money to fund Boris Johnson's legal defence during the Partygate inquiry.
The NAO is seeking further information and may carry out a formal investigation.
The Labour Party has criticised Rishi Sunak's decision to allocate taxpayers money to Johnson's defence and has urged Johnson to cover the bill himself.
The National Audit Office is looking into the government's decision to use £220,000 of taxpayers' money to fund Boris Johnson's legal defence during the Partygate inquiry.
The decision has been criticised by the Labour Party with Keir Starmer urging Johnson to cover the bill himself.
The issue continues to be debated on moral as well as legal grounds.
Tom Brake, director of the Unlock Democracy campaign group, wrote to the NAO to inquire whether the use of public money was a "sensible and legitimate use".
One of its directors has already requested a meeting with the Cabinet Office to obtain more information.
While Johnson maintains that he followed the rules during the pandemic, the allegations against him are serious and the NAO's investigation is bound to impact further judicial action.