Boris Johnson has said he is “more than happy” to hand over his unredacted WhatsApp messages and notebooks directly to the Covid inquiry, as the Government prepares for a legal battle with the probe.

The offer by the former prime minister came after the Cabinet Office took the highly unusual step of seeking a judicial review of inquiry chairwoman Baroness Hallett’s order to release the documents, arguing that it should not have to hand over material which is “unambiguously irrelevant”.

The move prompted swift criticism from bereaved families and opposition parties, but the Government has insisted that there are “important issues of principle at stake” affecting the rights of individuals and “the proper conduct of government”.

Mr Johnson, in his own letter to the inquiry on Thursday evening after the legal proceedings were launched, offered to hand over the requested material directly.

He wrote: “I agree with the Cabinet Office position that in principle advice to ministers should not be made public. That is clearly essential for the effective running of the country and for the impartiality of the civil service.

“I am simply making a practical point: that I see no reason why the inquiry should not be able to satisfy itself about the contents of my own Whatsapps (sic) and notebooks, and to check the relevant Whatsapp (sic) conversations (about 40 of them) for anything that it deems relevant to the Covid inquiry.”

The Government had faced a 4pm deadline on Thursday to hand over the material, which came after days of public wrangling between ministers and the inquiry.

In a letter to the inquiry, released after the deadline had passed, the Cabinet Office said it had provided “as much relevant information as possible, and as quickly as possible” in line with the order.

The letter said that the Cabinet Office was bringing the judicial review challenge “with regret” and promised to “continue to co-operate fully with the inquiry before, during and after the jurisdictional issue in question is determined by the courts”.

That question will centre on whether Lady Hallett’s inquiry has the power to force ministers to release documents and messages which the Cabinet Office believes are “unambiguously irrelevant” and cover matters “unconnected to the Government’s handling of Covid”.

Covid-19 pandemic inquiry
Baroness Hallett (UK Parliament)

In making the judicial review application, the Cabinet Office argues that concerns are “sharpened by the fact that irrelevant material contains ‘references to personal and family information, including illness and disciplinary matters’ and ‘comments of a personal nature about identified or identifiable individuals which are unrelated to Covid-19 or that individuals’ role in connection with the response to it’”.

Elsewhere, it is argued the inquiry’s concept of what is or is not relevant could have “absurd” implications and would leave the body “utterly swamped” and potentially slow proceedings.

A tranche of documents released on Thursday as part of the legal proceedings offer some new insights into the inquiry proceedings, including a list of 150 questions sent to Mr Johnson by the inquiry in early February.

Among the questions issued are: “In or around Autumn 2020, did you state that you would rather ‘let the bodies pile high’ than order another lockdown, or words to that effect? If so, please set out the circumstances in which you made these comments.”

He was also asked: “Between January and July 2020 did you receive advice from the then Cabinet Secretary that the Secretary of State for Health and Social Care, Matt Hancock MP, should be removed from his position? If so, why?”

In a statement to the inquiry, senior civil servant Ellie Nicholson also revealed that the WhatsApp messages passed to the Cabinet Office by Mr Johnson are only from May 2021 onwards.

Ms Nicholson said: “I understand that this is because, in April 2021, in light of a well-publicised security breach, Mr Johnson implemented security advice relating to the mobile phone he had had up until that time.”

Mr Johnson was forced to change his mobile in 2021 after it emerged his number had been publicly available online for 15 years.

A spokesman for the former prime minister said that Mr Johnson has “absolutely no objection whatsoever to providing content on the phone to the inquiry”.

“He has written to the Cabinet Office asking whether security and technical support can be given so that content can be retrieved without compromising security.

“The Cabinet Office have long been aware of the status of the phone.”

A spokesman for the Covid-19 inquiry said: “At 4pm today the chair of the UK Covid-19 public inquiry was served a copy of a claim form by the Cabinet Office seeking to commence judicial review proceedings against the chair’s ruling of May 22 2023.

“Further information will be provided at the module two preliminary hearing at 10.30am on June 6.”

The Government move has prompted warnings that bereaved families could now regard the public inquiry as a “whitewash and cover-up”.

Elkan Abrahamson, head of major inquests and inquiries at Broudie Jackson Canter – who represents the Covid-19 Bereaved Families for Justice group, said: “The Cabinet Office is showing utter disregard for the inquiry in maintaining their belief that they are the higher power and arbiter of what is relevant material and what is not.

“It raises questions about the integrity of the inquiry and how open and transparent it will be if the chair is unable to see all of the material.”

Lord Saville, who conducted the inquiry into Bloody Sunday, suggested he might consider quitting if he was in Baroness Hallett’s position.

“Trying to put myself in her position, if I was prevented from conducting a full and proper inquiry, I might seriously consider resigning on the grounds that I was unable to do a proper job”, he told Channel 4 News.

“It is a course that could be open her if she felt her efforts to carry out a thorough inquiry were being frustrated.

Labour’s Angela Rayner accused Mr Sunak of being “hopelessly distracted with legal ploys to obstruct the Covid inquiry in a desperate attempt to withhold evidence”.