Boris Johnson has been warned that he could lose public funding for legal advice if he tries to “frustrate or undermine” the government’s position on the Covid-19 inquiry. Cabinet Office lawyers told the former prime minister that money would “cease to be available” if he breaks conditions such as releasing evidence without permission, the Sunday Times reported. Johnson has been at the centre of a row as ministers launched a high court bid to challenge the inquiry’s demand for his unredacted WhatsApp messages and contemporaneous notebooks. He said he would send all his messages to the official investigation directly, circumventing the Cabinet Office. Last week, the Times reported that Johnson sent 300 pages of unredacted WhatsApp correspondence to the inquiry after the government began a judicial review designed to block the disclosure of his messages without prior vetting by officials and to ensure Whitehall has the final say on what is handed over. The Sunday Times detailed a letter sent by Cabinet Office lawyers to Johnson last week. “The funding offer will cease to be available to you if you knowingly seek to frustrate or undermine, either through your own actions or the actions of others, the government’s position in relation to the inquiry unless there is a clear and irreconcilable conflict of interest on a particular point at issue,” it said. The letter added that funding would “only remain available” if he complied with conditions such as sending the Cabinet Office “any witness statement or exhibit which you intend to provide to the inquiry so that it can be security-checked by appropriate officials”. The Cabinet Office said the letter was “intended to protect public funds” so that taxpayer-funded lawyers are not used for any purpose other than aiding the inquiry. Tory donor Lord Cruddas, an outspoken backer of Johnson, who handed him a peerage, urged the MP not to be “held to ransom” by the perceived threat. “Don’t worry, Boris Johnson, I can easily get your legal fees funded by supporters and crowdfunding, it’s easy,” he tweeted. Johnson wrote to the inquiry’s chair, Heather Hallett, saying he was sending all the unredacted WhatsApps he had given to the Cabinet Office. He added that he would like to send messages pre-dating April 2021, but that he had been told he could no longer access his phone from that period “safely”. Security concerns were raised over the phone in 2021 after it emerged that the number had been available on the internet for 15 years. The messages received before this date would cover discussions before May 2021 including those concerning the three national lockdowns he ordered. Johnson said he wanted to “test” the advice received from the security services and had asked the Cabinet Office for assistance in turning his old phone on securely. The former Conservative leader told the chairwoman that he was “not willing to let my material become a test case for others when I am perfectly content for the inquiry to see it”. The Cabinet Office missed Lady Hallett’s deadline set on Thursday to hand over the requested material. But the government department has been trying to resist the publication of messages it believes are “unambiguously irrelevant”. A Cabinet Office spokesperson said: “This letter from officials simply reiterates that taxpayer-funded lawyers must be used to aid the Covid inquiry and for no other purpose. The letter makes clear Mr Johnson has a duty to provide sincere witness to the inquiry independently and without reference to the views of the current government. “This letter was intended to protect public funds. It in no way prevents Mr Johnson from providing whatever evidence he wants to.” It comes after Rishi Sunak initiated a judicial review to block requests from Hallett to hand over his unredacted WhatsApp messages. Hallett’s demand relates to messages between Johnson and several officials during the pandemic, including Sunak, who was then chancellor, and other cabinet ministers. Labour’s deputy leader, Angela Rayner, told the Sunday Mirror: “In the middle of a cost-of-living crisis, taxpayers will rightly be outraged to be picking up yet another legal bill to pay for Rishi Sunak’s latest plot to obstruct the Covid inquiry.”