The Cabinet Office could take unprecedented action to prevent Boris Johnson’s unredacted diaries and WhatsApp messages being handed over to the official Covid inquiry, the Guardian had been told. Officials are preparing to issue a response to the inquiry chair, Heather Hallett, by 4pm on Tuesday. Sources said they were likely to resist her demand for a cache of documents relating to the former prime minister’s time in No 10. The news comes before an expected meeting later this week between Johnson and the prime minster, Rishi Sunak, which sources said would give the pair a chance to clear the air over the row about new Partygate evidence being handed to police. Lady Hallett has demanded the full cache of messages and diaries be handed over to her inquiry two weeks before the first public evidence sessions, but the government is refusing to comply. Lawyers for the Cabinet Office are said to have advised that the Covid inquiry does not have the powers to request access to all documents, raising the prospect of legal arbitration and a potential judicial review. Launching a legal challenge against the ruling by the head of a public inquiry would be unprecedented, sources said. Government insiders denied they would be delaying the next stage of the Covid inquiry. Hearings are expected to begin in two weeks on pre-pandemic preparedness, with former senior Tories including David Cameron and George Osborne giving evidence. Instead, the insiders said timings were a matter for the chair and that she could proceed whether or not she had all the evidence requested. They also said that handing over Johnson’s unredacted diaries and WhatsApp messages from the former prime minister and his aide Henry Cook would be an affront to their privacy and the right to private policy discussion. The row hinges on the Cabinet Office’s insistence that it will only hand over what it deems relevant, meaning many messages and diary entries have been redacted. It argued against Hallett’s request and said it would not hand over “unambiguously irrelevant material”. In a ruling issued last week, however, Hallett said: “The entire contents of the documents that are required to be produced are of potential relevance to the lines of investigation that I am pursuing.” Given that the terms of reference for her inquiry are so wide and were set by the government, she believes it is entitled to request such a vast trove of documents. The Cabinet Office will reply to Hallett’s demand by 4pm on Tuesday, but may not immediately lay out its next steps for contesting her ruling, sources indicated. The Lib Dem health spokesperson, Daisy Cooper, said failure to hand over everything requested would “make a mockery of this whole process and would be yet another insult to the millions of bereaved still waiting for justice”. She said Sunak appeared “too worried about upsetting Boris Johnson and his allies to do the right thing … The public deserves the whole truth about what went wrong. Vital evidence shouldn’t be kept secret just to spare ministers’ blushes.” It emerged last week that the Cabinet Office passed new allegations of Johnson’s wrongdoing to the police. They did so after seeing diary entries about guests who had visited Chequers during the pandemic, which Johnson handed to lawyers representing him as part of the Covid inquiry. Johnson has denied that the diaries show wrongdoing, and his allies have claimed he is the victim of a politically motivated stitch-up. No 10 has stressed that Sunak had no involvement in the decision to hand over Johnson’s pandemic diaries. After meeting Johnson later this week, Sunak will attempt to put the renewed focus on Partygate behind him when he flies to a meeting with other leaders at the European Political Community forum in Moldova. A spokesperson for the Cabinet Office said: “We are fully committed to our obligations to the Covid-19 inquiry. As such, extensive time and effort has gone into assisting the inquiry fulsomely over the last 11 months. “We will continue to provide all relevant material to the inquiry, in line with the law, ahead of proceedings getting under way.”