The government is set to launch an unprecedented legal challenge to the Covid inquiry’s demand for WhatsApp messages and documents.

Authorities have missed a 16:00 deadline to release communications between Boris Johnson and his advisers during the pandemic, as well as his diaries and memos.

It is believed to be the first time a government has taken legal action against an inquiry it has set up.

Mr Johnson said he was “happy” to hand over unredacted material if asked.

In a letter to Baroness Hallett, chair of the Covid inquiry, Mr Johnson said: “If you would like to receive this item immediately, please let me know where and how you would like it sent to you.”

A spokesman for the former prime minister said a mobile phone used by Mr Johnson during a critical period of the pandemic was involved in a security breach and had not been switched on.

In April 2021, a year after the first lockdown was introduced in the UK, Mr Johnson’s mobile number was traced online for more than 15 years. The contents of the phone have not been seen by the Covid investigation, the BBC said.

Mr Johnson has written to the Cabinet Office asking if it can provide security and technical support to restore content without compromising security, the spokesman added.

The government has refused to release some of the information, arguing that it is not relevant to the investigation’s mission.

But the inquiry’s boss says it should be her job to decide what’s appropriate.

Crossbench peer Baroness Hallett says the news should be looked at to see if they are relevant to the inquiry’s inquiry into how the government handled the pandemic.

The government says handing over the requested materials will set a precedent that prevents ministers from discussing policy matters in the future.

The Cabinet Office, which heads the government, has said it will apply for a judicial review. This means that the judge will decide whether the investigation violates statutory powers to subpoena evidence.

Elkan Abrahamson, a lawyer representing families bereaved by Covid-19 for the Justice Committee, said: “The Cabinet Office completely ignores the inquiry in maintaining their belief that they are the supreme authority and the arbiter of what is and what is not. .

“It raises questions about the integrity of the inquiry and how open and transparent it can be if the chair can’t see all the material.”

The opposition has accused Rishi Sunak’s government of trying to block the Covid probe and urged him to comply with its demands.

Labor deputy leader Angela Rayner said: “These latest smoke and mirrors tactics only serve to undermine the Covid inquiry.”

The Liberal Democrats said the legal challenge was “a kick in the bum for grieving families who have already waited too long for answers”.

Some senior Conservative MPs urged the government to back off to avoid a lengthy legal battle with the Covid inquiry.

The “inappropriate content” requested by the inquiry included “references to personal and family information, including illness and disciplinary matters” and “comments of a personal nature about identified or identifiable individuals unrelated to Covid-19”.

The legal challenge came a day after Mr Johnson said he had handed the Cabinet Office all WhatsApp messages and memos requested by the Covid inquiry.

He urged the Cabinet Office to submit it to the inquiry in its entirety, without any changes, and said he would do it himself “if asked”.

The inquiry sought access to WhatsApp messages from Mr Johnson’s phone, covering the period from 1 January 2020 to 24 February 2022.

But the content submitted by Mr Johnson does not include messages sent before May 2021.

This was because he was forced to change phones after the security breach, the director of the Cabinet Office said in a statement to the inquiry.