Inquiry chair Baroness Hallett issued an order - known as a section 21 notice - for the government to release the former prime minister's documents in May.

The Cabinet Office argued some of the content was "unambiguously irrelevant" and sought a judicial review against the notice.

But in a judgment published on Thursday, the High Court ruled while "some irrelevant documents" may be included in the material requested, that "does not invalidate the notice or mean that the section 21 cannot be lawfully exercised".

It dismissed the Cabinet Office's claim for judicial review, but said but said the department could make a different application to Baroness Hallett.

The material must now be handed to the inquiry by 4pm on Monday.

The government said it will "comply fully" with the High Court judgment.

A spokesperson said: "The inquiry is an important step to learn lessons from the pandemic and the government is cooperating in the spirit of candour and transparency.

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"As this judgment acknowledges, our judicial review application was valid as it raised issues over the application of the Inquiries Act 2005 that have now been clarified.

"The court's judgment is a sensible resolution and will mean that the inquiry chair is able to see the information she may deem relevant, but we can work together to have an arrangement that respects the privacy of individuals and ensures completely irrelevant information is returned and not retained."

'A humiliating defeat'

Responding to the ruling, Labour's Deputy Leader Angela Rayner said: "While the rest of the country battles the cost-of-living crisis, Rishi Sunak has been wasting time and taxpayers', money on doomed legal battles to withhold evidence from the COVID Inquiry.

"After this latest humiliating defeat, the prime minister must accept the ruling and comply with the inquiry's requests for evidence in full.

Deborah Doyle, spokesperson for COVID-19 Bereaved Families for Justice UK, said the judicial review lodged by the government "was a desperate waste of time and money".

"The inquiry needs to get to the facts if the country is to learn lessons that will save lives in the future," she said. "That means it needs to be able to access all of the evidence, not just what the Cabinet Office wants it to see.

"A successful inquiry could save thousands of lives in the event of another pandemic, and it's a disgrace that the Cabinet Office is trying to obstruct it. Any attempt to appeal this decision or hinder its work further would be utterly shameful."

In their ruling, Lord Justice Dingemans and Mr Justice Garnham concluded Baroness Hallett had "acted rationally" when making her order.

Addressing the concern that unrelated material could be seen by the inquiry, the two judges noted that the Cabinet Office could make an application arguing "that it is unreasonable to produce material which does not relate to a matter in question at the inquiry" - and that it would be for the chair to "rule on that application".

Throughout the process, Mr Johnson backed the COVID inquiry's attempts to see his unredacted messages, saying he "no objections" to the inquiry seeing his entire back catalogue of communications.

In June he decided to bypass the Cabinet Office by sending "all unredacted WhatsApps" directly to the COVID inquiry, saying he was "perfectly content" for the material to be inspected.

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The former prime minister said he would "like to do the same" with texts that are on an old mobile phone he stopped using due to security concerns in May 2021 - more than a year after the pandemic began.

He said he had asked the government for its help to turn on the device securely to hand over the material.