Ministers and Whitehall officials have until 4pm tomorrow to hand over WhatsApp messages - including from Rishi Sunak and Boris Johnson - to the Covid-19 inquiry.
The Government was set the crunch deadline by probe chairwoman Baroness Hallett.
Insiders admit they fear setting a precedent by surrendering the texts - which could reveal behind-the-scenes infighting and distractions at the height of the pandemic.
Ministers are due to make a decision this evening on whether comply or wage a bid to keep some of the WhatsApps private.
The row comes after the Cabinet Office handed extracts from Mr Johnson's prime ministerial diaries which showed potential lockdown breaches to the police without telling him.
The entries showed visits by friends to Chequers - the Prime Minister's grace and favour country residence - and further events in Downing Street.
Mr Johnson, who was fined for attending a gathering in Downing Street to mark his birthday in June 2020, has said the diary entries "merely record events in my day".
The former PM is claiming that handing over the diaries would breach government rules on disclosure and has refused to provide the messages on national security grounds, according to the Times.
The Cabinet Office said ministers played "no role" in the decision-making process behind the diary entries being referred to the police, while Mr Johnson believes he is the victim of a "politically motivated stitch-up".
Baroness Hallett, the chair of the Covid inquiry, has issued a legal notice to the Cabinet Office for not handing over the full content of messages sent between the ex-PM and dozens of high-profile figures, including then-Cabinet ministers Rishi Sunak and Liz Truss, and No10 advisers during the pandemic.
The inquiry had issued a "section 21" notice in April requesting the files - but the department refused, arguing some of the information is "irrelevant".
Rejecting the argument, Baroness Hallett has now given the Government until 30 May to provide the material - or face the prospect of legal action.
The Cabinet Office has so far released more than 55,000 documents, 24 personal witness statements and eight corporate statements to the inquiry.
The government believes it is not obliged to provide "unambiguously irrelevant" material.
However Baroness Hallett rejects their argument about them deciding what or what isn't "unambiguously irrelevant".
She said all these documents "contain information that is potentially relevant" to decisions-making processes in the pandemic.
Mr Johnson wrote to the inquiry claiming that he was not the one to blame for the redactions, and that the order "may directly and/or indirectly suggest that I have failed to provide documents to the inquiry".
The former PM has sought out new lawyers since the row broke over his diary entries - for which taxpayers are set to be billed more than a million pounds.
Labour's Deputy Leader Angela Rayner has said: “The public will be shocked that they're still paying Boris Johnson’s legal bills while he rakes in millions from globe-trotting speaking gigs.
"Having apparently sacked lawyers already paid for by the taxpayer, it would be outrageous for him to expect the public to pick up yet another eye-watering new bill for his own private legal team.
"His shameless demands have been tolerated by the Prime Minister for far too long. Rishi Sunak must make it clear that he will put a stop to this unacceptable arrangement once and for all.”
A Cabinet Office spokesman said: "Information came to light during the process of preparing evidence for submission to the Covid Inquiry. It was identified as part of the normal disclosure review of potentially relevant documents being undertaken by the legal team for inquiry witnesses. In-line with obligations in the Civil Service Code, this material has been passed to the relevant authorities and it is now a matter for them."
He added: “The Cabinet Office has not made any assessment or conducted any investigation of the material that has been passed to the police. Ministers played no role in deciding whether the information should be handed over to the police.
“The police were first contacted on 16th May prior to any Minister being made aware. The decision to contact the police and the subsequent decision to share the information was not made by Ministers but by officials acting in line with the Civil Service Code."
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