Britain's former leader Boris Johnson urged a court on Friday to allow the release of potentially embarrassing WhatsApp messages between senior figures to a Covid-19 inquiry despite objections by his successor Rishi Sunak's government.

Johnson, who ordered the inquiry in 2021 after Britain recorded one of the world's highest death tolls from the pandemic, resigned last year due to scandals including repeated breaches of Covid-19 lockdown rules.

He has been at odds with his former ally Sunak since then, dividing the ruling Conservatives to the delight of the opposition Labour Party which believes it can win the next election, due by early 2025.

The Covid inquiry's chair Heather Hallett had asked the Sunak government to provide two years' worth of messages, including from Johnson and 40 different individuals.

But the Cabinet Office earlier this month refused to reveal certain WhatsApp messages about the pandemic and other political issues, saying some material sought was "unambiguously irrelevant" and the request was beyond Hallett's remit.

Johnson asked London's High Court to reject that, showing the political problems he poses to his former friend.

His lawyer David Pannick, who also gave Johnson advice over Parliament's "Partygate" inquiry into illegal gatherings, said on Friday that the former prime minister backed Hallett's stance.

"In establishing the inquiry, Johnson said... he wanted the state's action to be placed 'under the microscope' and that the inquiry must be 'free to scrutinise every document'," Pannick added in court filings.

"That is what the public expects and that is what should be done."

The Cabinet Office's lawyer James Eadie earlier told the court the case was brought with reluctance.

He argued that the documents relevant to the inquiry "simply cannot cover all government business and all the policy areas that were live over the two-year period".

Eadie said the WhatsApp messages contained references to personal and family information and "comments of a personal nature" about identifiable government figures.

Lawyers representing Hallett said limiting public inquiries' powers to see documents would "emasculate" them and enable the Cabinet Office to "mark its own homework."

One of those lawyers, Hugo Keith, said in court filings that a witness statement provided by Johnson earlier this month led to WhatsApp messages redacted by the Cabinet Office as being "unambiguously irrelevant" being reassessed as relevant.