Boris Johnson will not be restricted by the government over what he can say to the Covid-19 inquiry, despite being warned he should ensure he is using public funding for legal advice “appropriately”, a senior minister has said. Robert Jenrick’s comments on Sunday come after it was reported that Cabinet Office lawyers had told Johnson he could lose the funding if he tried to “frustrate or undermine” the government’s position on the inquiry. The former prime minister was informed that the money would “cease to be available” if he broke conditions such as releasing evidence without permission, according to the Sunday Times. 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. Jenrick told Sky’s Sophy Ridge on Sunday that Johnson – in whose administration he served and who he backed for the Conservative leadership – was “at liberty” to send his documents or WhatsApp messages to the inquiry and make whatever statements he wishes. But the immigration minister added: “There’s absolutely no sense that the government will restrict what Boris Johnson wants to say, but if you use taxpayer funds, obviously you should make sure you’re using them appropriately.” He denied there were fears in No 10 that Rishi Sunak’s messages could reveal a plot to try to bring down Johnson, insisting it would not be “sensible or reasonable” to hand over ministers’ documents or messages if they were deemed irrelevant to the pandemic. “I can’t think of a situation where a court would ask for documents or messages that have absolutely nothing to do with the purpose of the case or the inquiry. I mean, for example, things to do with a civil servant or a politician’s private life that are wholly unrelated to the inquiry,” he said. Jenrick, a former lawyer, said the “normal way to do this is to set reasonable parameters” and not to ask for things “wholly unrelated”. He insisted the government had the “highest regard” for the inquiry chair, Heather Hallett, and was not asking for “special treatment”. “I hope this can be resolved indeed even before the matter gets to court,” he added. A former director of the Governments Communications Headquarters (GCHQ) has called for rules over the use of WhatsApp in government and said ministers “should think twice before they express their views”. David Omand told Times Radio “We do need some rules in this space. For example, classified material security classified material should not be discussed between ministers on social media in that way.” Omand, who was the UK’s first security and intelligence coordinator, also said the government “ought to make the material available” to Lady Hallett. “I don’t buy the argument that she shouldn’t see material that the government simply says is not relevant, because she has wide-ranging terms of reference,” he said.