The Cabinet Office failed to follow proper processes when it allowed taxpayers’ money to be used to fund Boris Johnson’s Partygate legal bills, the UK’s public spending watchdog has said. The government’s justifications for the £265,000 spend were also deemed to be “borderline” and not “wholly persuasive” by the National Audit Office (NAO). In a report that will cause embarrassment inside the Cabinet Office, Gareth Davies, head of the NAO, said its audit team had exerted “significant effort” to investigate whether the spending was “a legitimate use of public money”. The decision allowed Johnson to have his legal fees during the privileges committee inquiry into his Partygate denials covered by the taxpayer. It was criticised at the time by some, including the Labour party, which said that given Johnson should have paid the bill himself given he was making millions from his post-Downing Street career. Johnson was found to have committed five contempts of parliament after the year-long investigation into whether he misled parliament by denying any Covid rules were broken in No 10 during lockdown. After the Guardian revealed in February that the NAO was examining the decision, the findings of its long-awaited audit were published on Tuesday. The then-permanent secretary in No 10, who was not named in the report but is understood to be Samantha Jones, failed to follow proper processes when the initial contract for £129,000 was awarded last August, said the NAO. Though Jones got assurances from the propriety and ethics team and others inside government, the NAO said the decision should have been taken by a designated accounting officer. Such an officer was only brought into the process the following month. After Jones left the civil service, future increases to the spending were authorised by the proper civil servant in the Cabinet Office. The Cabinet Office’s argument that there was precedent for spending public money defending serving or former ministers was also questioned by the NAO. There was “no case that is exactly analogous to the circumstances”, the Cabinet Office admitted in its submissions to the NAO. But the Cabinet Office maintained the government’s approach was “in line with established precedent set by both public inquiries and litigation”. The examples highlighted by the government were “substantively different” from the case involving Johnson, said the NAO. The Cabinet Office denied any incorrect processes were followed, and said there was an “established precedent” that ministers of all political colours could get legal support after they had left office when matters related to their time and conduct as a member of the government. A spokesperson for the Cabinet Office said: “The government has been consistently clear that the contract award followed the proper procurement process.” The audit was revealed in the Cabinet Office’s annual accounts, which were signed off by the NAO. Whitehall sources pointed to the Treasury agreeing the spending on Johnson’s legal fees was “regular” as further evidence all the correct processes were followed. It was also revealed this week that Liz Truss had claimed £23,310 of her allowance as a former prime minister – despite having only lasted in post for 49 days.