The government has until 5 June to launch legal action against the Covid inquiry chair, Heather Hallett, in order to keep the material secret.

Legal sources say that failing to comply with Hallett's order could result in a criminal offense.

The Covid inquiry is set to investigate the government's preparations for and response to the pandemic, and will include prominent Tories from the past 13 years.

Senior figures including Dominic Grieve and Charles Falconer have warned that any legal challenge by the government is unlikely to succeed.

The legal standoff between the Covid-19 inquiry and the Cabinet Office has intensified, with the inquiry using its powers to demand unredacted documents.

The Cabinet Office has refused, claiming only "unambiguously irrelevant" material was removed, but the inquiry's demand has been upheld by the judge.

The inquiry has also seen information regarding discussions between Johnson and his advisers about the Metropolitan police's enforcement of Covid regulations following the murder of Sarah Everard.

The Cabinet Office has told the Observer that it is sticking by its position of only providing relevant material.

Johnson and the Cabinet Office have said they will continue to provide all relevant material to the inquiry.