The chair of the COVID inquiry says it's as much as her to determine what proof is "relevant or potentially relevant" amid a authorized row with the federal government over Boris Johnson's WhatsApp messages.

Baroness Hallett refused to withdraw her order for the federal government at hand over unredacted materials for her investigation as she formally opened the COVID inquiry on Tuesday.

It comes simply days after the federal government launched a judicial evaluation over her order to the Cabinet Office that it hand over Mr Johnson's unredacted WhatsApp messages and different paperwork.

The former prime minister has already sent "all unredacted WhatsApps" on to the inquiry.

Acknowledging the authorized battle, Baroness Hallett mentioned: "As has been extensively reported within the media, a problem has arisen between the inquiry and the Cabinet Office as to who decides what's related or doubtlessly related.

"I issued a notice under Section 21 of the Inquiries Act 2005 making it clear that, in my view, it is for the inquiry chair to decide what is relevant or potentially relevant.

"The Cabinet Office disagrees, claiming they aren't obliged to reveal what they contemplate to be unambiguously irrelevant materials. They invited me to withdraw the Section 21 discover. I declined.

"They are now challenging my decision to decline to withdraw the notice in the High Court by way of judicial review.

"With litigation pending and because the decision-maker, I could make no additional remark."