Ministers are under pressure over whether to release unredacted WhatsApp messages and diaries belonging to Boris Johnson to the Covid-19 inquiry or risk sparking a further legal row.
The Cabinet Office has until 4pm on May 30 to respond to the request or risk the prospect of legal action.
It is so far resisted the request from Lady Hallett’s official inquiry, raising the possibility that ministers could seek to challenge it by way of judicial review.
The row was sparked by a legal request sent by the inquiry on April 28 for a number of materials, including unredacted WhatsApp messages and diaries belonging to the former prime minister between January 2020 and February 2022.
In May the Cabinet Office pushed back against the request, which was made under section 21 of the Inquiries Act 2005 and which also applies to messages from former adviser Henry Cook.
In a ruling last week, Lady Hallett rejected the argument that the inquiry’s request was unlawful and claimed that the Cabinet Office had “misunderstood the breadth of the investigation”.
In her response, she said that the requested documentation was of “potential relevance” to the inquiry’s “lines of investigation”.
The Cabinet Office has already provided more than 55,000 documents, 24 personal witness statements and eight corporate statements to the inquiry.
But the Government believes it has no duty to disclose “unambiguously irrelevant” material.
A Cabinet Office spokesman 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.”
According to the notice seeking the unredacted messages, the inquiry is requesting conversations between Mr Johnson and a host of government figures, civil servants and officials.
The list includes England’s chief medical officer Professor Sir Chris Whitty, as well as then-chief scientific adviser Sir Patrick Vallance.
Messages with then-foreign secretary Liz Truss and then-health secretary Matt Hancock are also requested, as well as with former top aide Dominic Cummings and then-chancellor Rishi Sunak.
The inquiry had also asked for “copies of the 24 notebooks containing contemporaneous notes made by the former prime minister” in “clean unredacted form, save only for any redactions applied for reasons of national security sensitivity”.