The government's spending watchdog is looking into whether billing taxpayers £220,000 defending Boris Johnson over the Partygate scandal is money well spent.
It emerged this week that legal fees had rocketed as the former Prime Minister battles allegations that he misled Parliament.
Labour has piled pressure on Rishi Sunak to "stand up" to Mr Johnson and make him pay for it himself.
Keir Starmer taunted him at PMQs this week, saying: "Surely even this Prime Minister can put his foot down and stand up to his old boss and say he made this mess, he can pick up the bill?"
The National Audit Office (NAO) has now confirmed it is "seeking further information" sand would be considering the matter as part of its audit.
No decision has been made on whether to launch a formal investigation.
It follows a request by campaign group Unlock Democracy to look at whether the spending is a "sensible and legitimate use of public money".
The Mirror previously revealed the estimated cost in defending the shamed former Prime Minister had soared by more than £90,000 since last summer.
Mr Johnson, now a backbench MP, faces a probe over his denials of lockdown-busting gatherings in No10, with hearings likely to begin next month.
He is being investigated by the House of Commons Privileges Committee, which will determine whether he committed contempt of Parliament - an allegation he denies.
Solicitors firm Peters and Peters were awarded a contract worth £129,700 to provide Mr Johnson with advice during the investigation.
Cabinet Office permanent secretary Alex Chisholm confirmed to MPs that the sum had risen dramatically, and could not guarantee it wouldn't go up even further.
An NAO spokesperson said: “The NAO received a query from a member of the public relating to our audit of Cabinet Office accounts.
"We are seeking further information in response to the correspondence and will be considering the matter as part of our audit.”
It has raised questions over whether Mr Johnson should be footing the bill himself, having raked in millions from speeches and an advance on his memoirs since leaving office.
Former Lib Dem MP Tom Brake, who heads Democracy Unlocked, told The Guardian: “In these circumstances, and with public finances under huge pressure, the taxpayer shouldn’t underwrite his open-ended defence legal costs.
"Fortunately for Johnson, thanks to his recent extra-parliamentary activities, he can afford to pay for them himself.”
The NAO told Democracy Unlocked that it had been in touch with the Cabinet Office, The Guardian reports.
The cost of the legal defence has sparked huge anger.
Liberal Democrat Chief Whip Wendy Chamberlain previously said: "While the British people battle with a cost-of-living crisis, this Conservative Government seems more interested in helping Boris Johnson with his cost-of-lying crisis. This is a sleazy new low for this Government, dragging politics into the gutter."
Mr Johnson has denied any wrongdoing, telling close ally Nadine Dorries in an interview set to be screened tomorrow: "As you know there’s a Parliamentary Committee looking into some aspects of this and I had better be respectful of them.
"But I’ll just repeat what I've said before, and I hope it's obvious to everybody, that anybody who thinks I was knowingly going to parties that were breaking lockdown rules in No10, and then knowingly covering up parties that were illicit that other people were going to, that’s all strictly for the birds.
"And if anybody thinks like that they’re out of their mind.”
