The Cabinet Office is seeking a judicial review of Baroness Hallett’s order to release Boris Johnson’s WhatsApp messages, diaries and notebooks, it has said in a letter to the COVID inquiry.

The Cabinet Office, in a letter to the inquiry, said it was bringing a judicial review challenge “with regret” – but added that there were “important issues of principle at stake”.

It said it would “continue to co-operate fully with the inquiry before, during and after the jurisdictional issue in question”.

The government has so far refused to hand them over in their entirety to the official inquiry – and the deadline to do so expired at 4pm today.

Outlining its refusal to hand over the material in a letter to the inquiry, the Cabinet Office questioned whether the public inquiry has “the power to compel production of documents and messages which are unambiguously irrelevant to the inquiry’s work”.

Such materials include personal messages and “matters unconnected to the government’s handling of COVID” it said.

It argued that requesting “irrelevant” material “represents an unwarranted intrusion into other aspects of the work of government”.

“We consider there to be important issues of principle at stake here, affecting both the rights of individuals and the proper conduct of government,” the department said.

It said it has explored “a number of possible avenues for resolving this difference of opinion”, including offering redacted versions of the materials and “a more focused or sequential approach to the direction of the information requirements”.

“We remain hopeful and willing to agree together the best way forward,” it added.

The Cabinet Office went on to say that while it had chosen a judicial review over providing the documents requested by the inquiry, it nevertheless wanted to “provide as much relevant information as possible, and as quickly as possible, to assist the work of the inquiry”.

Inquiry chair Lady Hallett ordered the government to hand over the documents last week – without any amendments.

The tranche includes messages sent between former prime minister Mr Johnson and his fellow ministers and advisers during the pandemic, as well as diary entries.

On Wednesday afternoon, Mr Johnson’s team said all the information the inquiry wanted had been passed to the Cabinet Office.

His spokesman said the ex-prime minister wanted the Cabinet Office to “urgently” hand over the material.