Inquiry chair Lady Hallett had ordered the government to hand over the messages - alongside diary entries and notes - by 4pm on Tuesday 30 May.

The deadline has now set been 4pm on Thursday 1 June. The Cabinet Office asked for an extension to Monday 5 June as they do not have access to Mr Johnson's messages or notebooks, but this was rejected.

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This is despite saying in their original appeal against the order that there was "unambiguously irrelevant" material in the redacted parts of messages sent to the inquiry.

When the Cabinet Office lodged the appeal on 15 May, it said Mr Johnson's WhatsApp messages had not yet been received by the government.

A spokesman for the former prime minister said today that he had "no objection" to sending the material to the inquiry.

Mr Johnson has written to the Cabinet Office to demand the government requests in writing access to his messages and notes - which he says has not happened yet.

Sources close to Mr Johnson say Cabinet Office officials have visited his office in person to examine notebooks within the past few weeks.

If the government does not abide by the new deadline on Thursday, Lady Hallett has ordered that a statement be sent by a "senior civil servant" confirming the Cabinet Office does not have the requested information, as well as a chronology of the government's contacts with Mr Johnson about the requests and whether the government has ever had the data.

Breaking a section 21 order could see the government face criminal proceedings, and there is also potential for a court battle over whether the information should be passed to the inquiry.

Speaking shortly before the inquiry's announcement Prime Minister Rishi Sunak said the "government is carefully considering its position, but it is confident in the approach that it's taking".

The battle between the parties centres on messages Mr Johnson sent and received, as well as his diaries and his notebooks from during the pandemic.

Lady Hallett made an order under section 21 of the Inquiries Act 2005 for the material to be handed over by the Cabinet Office.

Messages to and from former adviser Henry Cook were also included in the legal action.

It is this order which has now been extended.

Lady Hallett highlighted in her explanation that the Cabinet Office redacted material about the policing of Sarah Everard protests during the period of restrictions - something she said was "not a promising start".

She later obtained the messages in full.

She also identified communications - which have still only been seen in their redacted form - that she considers are "in fact relevant to my investigation".

Labour's deputy leader, Angela Rayner, said: "It now appears that vital evidence has gone missing. It must be found and handed over as requested if the whiff of a cover-up is to be avoided and bereaved families are to get the answers they deserve.

"It is for the COVID inquiry itself rather than Conservative ministers to decide what is and is not relevant material for its investigation, and this interference only serves to undermine the inquiry's crucial job of getting to the truth."

Daisy Cooper, the Liberal Democrats' deputy leader, added: "This dog ate my homework type excuse from the government simply doesn't stand up to scrutiny.

"For the Cabinet Office to simultaneously refuse to disclose Boris Johnson's messages because they were irrelevant to the inquiry, whilst claiming not to even have them, will raise suspicions of another Conservative cover up.

"Rishi Sunak is too weak to stand up to Johnson and make him hand over this evidence, while bereaved families are being left aghast.

"The public has waited long enough already to get the truth. The inquiry's work mustn't be delayed any longer because of endless chaos in the Conservative Party."