Officials at the government’s spending watchdog are examining the controversial decision to provide £220,000 of taxpayers’ money to fund Boris Johnson’s legal defence for the inquiry into his Partygate denials. The National Audit Office (NAO) has yet to decide whether to mount a formal investigation, but one of its directors is planning to speak to the Cabinet Office about it. On top of the six-figure budget already established, sources have also indicated more money could be set aside to cover the former prime minister’s legal advice, given the privileges committee’s investigation could drag on into next month. The revelations came as Johnson issued a fresh defence of his actions as PM during lockdown, insisting: “I thought what we were doing was within the rules.” Keir Starmer laid into Johnson for relying on taxpayers to foot the bill incurred during a long-running investigation into whether he misled parliament by denying any Covid rules were broken in No 10, before swathes of fines for illegal parties were handed out. The Labour leader said he “couldn’t quite believe it” and urged Rishi Sunak at prime minister’s questions on Wednesday to “put his foot down” and tell Johnson that as “he made the mess, he can pick up the bill”. The Cabinet Office gave the green light to an initial £129,000 contract for legal advice in August 2022 with the firm Peters & Peters, which Johnson has used to help defend himself since the start of the privileges committee’s investigation last summer. Due to delays getting the government to hand over information requested by the seven-member committee, which has a Conservative majority but a Labour chair, the contract has been extended until 28 February and the budget raised to £220,000. Tom Brake, the director of Unlock Democracy and a former Liberal Democrat MP, wrote to the NAO asking if it was within its remit to investigate whether the spending had been a “sensible and legitimate use of public money”. In a reply seen by the Guardian, the NAO director with responsibility for the financial audit of the Cabinet Office said they had contacted the the department to “request a meeting to obtain more information about any arrangements for the legal services”. The meeting was scheduled to take place this week, and the NAO director said they would review any relevant documentation provided by the Cabinet Office and provide a fuller response afterwards. Angela Rayner, Labour’s deputy leader, said it “rings alarm bells” and accused Sunak’s government of “writing a blank cheque for the disgraced prime minister’s legal fund”. She added: “Ministers must come clean on the nature of this murky legal contract, and explain both who this arrangement was agreed by and why it has been allowed to continue unchecked.” Brake said there was “strong evidence” Johnson misled parliament over Partygate. He 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.” An NAO spokesperson said a query had been received from a member of the public relating to its 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,” they added. The Cabinet Office defended the idea of taxpayers footing Johnson’s legal advice costs, saying: “There is an established precedent across multiple administrations that former ministers may be supported with legal representation after they have left office when matters relate to their time and conduct as a minister.” Sources have indicated that the £220,000 budget for Johnson’s legal advice could be increased again, given the contract expires on 28 February but the privileges committee has yet to finish collecting all oral evidence, begin public hearings or start writing its report. Johnson on Wednesday said he was being “respectful” of the privileges committee, as it continues to look into his denials from the dispatch box that any Covid rules were breached before confirmation of law-breaking parties came from Scotland Yard. He gave a rare interview to one of his fiercest supporters, the Conservative MP Nadine Dorries, for a new show on TalkTV she will front. Johnson said: “Anybody who thinks I was knowingly going to parties that were breaking lockdown rules in No 10, 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.” He admitted he had to “wait for this thing to conclude”, and added: “What I would say is that we all thought what we were doing – or certainly, I thought what we were doing – was within the rules. And what we certainly thought was that we were working blindingly hard on some massive priorities for the country.” Pointing to the government’s work during Covid, Johnson said he had been focused on “getting that vaccine rollout organised” and “thinking desperately about how to … ramp up testing and all the rest of it”. After leaving No 10, Johnson has seen his income rocket due to joining the speaking circuit, an advance payment for his memoirs and other visits and articles. Since October 2022, he has declared earnings of £2,296,905 – on top of his MP’s salary.