Ministers have launched an unprecedented high court attempt to avoid handing over Boris Johnson’s unredacted WhatsApp messages and diaries to the government-commissioned public inquiry into the handling of Covid. In a move immediately condemned by bereaved families and opposition MPs, the Cabinet Office told the inquiry, headed by the retired judge Heather Hallett, that there were “important issues of principle” over passing on information that might not be relevant. Johnson said on Wednesday he had given unredacted material to the Cabinet Office and urged it to share it with the inquiry. But the department is concerned this would set a precedent that could lead to Lady Hallett demanding the WhatsApp messages of serving ministers – not least the prime minister, Rishi Sunak. Another layer of potential chaos was added on Thursday evening when Johnson wrote personally to Hallett to say he could pass on his unredacted messages and emails directly, if it helped the inquiry. This would take place “without any prejudice to the judicial review”, the letter said, with Johnson adding that he understood the need for some parts of decision-making to be private. His offer was just to make a “practical point” that the inquiry should be able to see the material itself, he said. A spokesperson for the inquiry said Hallett was formally served notice of the Cabinet Office’s plan to seek judicial review of her demand for the material at 4pm on Thursday, the deadline she had set for the evidence to be provided. The Cabinet Office’s letter to the inquiry said the legal process was being launched “with regret and with an assurance that we will continue to cooperate fully with the inquiry before, during and after the jurisdictional issue in question is determined by the courts”. Hallett first demanded the WhatsApp messages and notebooks from Johnson, and texts from one of his No 10 aides, Henry Cook, last month. The Cabinet Office’s letter on Thursday said it had provided “as much information as possible”, including WhatsApp messages from Johnson and Cook redacted for national security reasons and to remove “unambiguously irrelevant material”. The letter said that while Johnson had provided the Cabinet Office office with some messages, these only dated from May 2021, when he acquired a new phone, and also the month he ordered the inquiry. Officials had asked the former prime minister to provide earlier WhatsApp communications, it added. Johnson’s contemporaneous diaries and notebooks would also be provided, with similar editing, the letter said, a process likely to be completed in the next two days. Hallett has insisted that she should decide what material is relevant to the inquiry. The Guardian was told that Sunak and Oliver Dowden, the deputy prime minister, signed off the decision to launch a judicial review on Wednesday. An ally of Johnson said the reason the former prime minister had not handed over pre-May 2021 messages was that he had changed phones after it emerged his number had been listed online, and that he was given security advice to not turn on the old device again. Johnson was thus unable to see the earlier messages, but he was happy to pass the phone to the inquiry if needed, the ally said. He had asked the Cabinet Office about possible security and technical support with this. The Cabinet Office was aware of this, the ally added. The prospect of a legal battle with a formal public inquiry is highly unusual and risks claims of a cover-up. When ministers set up the inquiry, they promised it would allow bereaved relatives and others to learn the truth about the decision-making process during the pandemic. The Covid-19 Bereaved Families for Justice group said the prospect of the Cabinet Office suing the inquiry was “absolutely obscene”. Rivka Gottlieb, a spokesperson for the organisation said: “Why are the Cabinet Office standing in their way? You have to assume that they’re sitting on evidence that will devastate Rishi Sunak’s reputation and that’s more important to them than saving lives in the future.” Angela Rayner, Labour’s deputy leader, said the public “deserve answers, not another cover-up”. She added: “Instead of digging himself further into a hole by pursuing doomed legal battles to conceal the truth, Rishi Sunak must comply with the Covid inquiry’s requests for evidence in full. There can be no more excuses.” Daisy Cooper, the Liberal Democrats’ deputy leader, said: “This cowardly attempt to obstruct a vital public inquiry is a kick in the teeth for bereaved families who’ve already waited far too long for answers.” While an interim decision on the issue could be made within days, which could potentially suspend Hallett’s demands while the case is heard, a full hearing could take weeks to be completed. The lengthy Cabinet Office letter said the issue to be determined by the high court was “whether the inquiry has the power to compel production of documents and messages which are unambiguously irrelevant to the inquiry’s work, including personal communications and matters unconnected to the government’s handling of Covid”. It went on: “We consider there to be important issues of principle at stake here, affecting both the rights of individuals and the proper conduct of government. The request for unambiguously irrelevant material goes beyond the powers of the inquiry … It represents an unwarranted intrusion into other aspects of the work of government. It also represents an intrusion into their legitimate expectations of privacy and protection of their personal information.” The letter was accompanied by a legal document setting out the claim, citing the Cabinet Office as the claimant, the inquiry as the defendant, and Johnson and Cook as “interested parties”. The decision to challenge the inquiry has already caused disagreement within government. Grant Shapps, the energy secretary, said officials should be given “whatever they want” and that there was nothing for ministers “to be shy or embarrassed about”. Jonathan Jones, who used to run the government legal department, told the Guardian he thought the courts would ultimately favour Hallett, if the government stuck to arguing on relevance grounds.