The chair of the Covid Inquiry could be forced to quit if the Government blocks her demands for unredacted WhatsApps, a lawyer representing bereaved families has warned.
Elkan Abrahamson said the "existential struggle" could leave Baroness Heather Hallett with no choice but to step down if the Government wins its legal battle over disclosure of evidence to the Inquiry.
After days of increasingly bitter wrangling, the Cabinet Office sought a judicial review last week over the Inquiry's demands for ministers' WhatsApps during the pandemic.
The Government argues this could lead to irrelevant personal information being handed to the inquiry but Baroness Hallett has said it is up to her to decide what is relevant.
WhatsApp messages could help the inquiry determine how focused Boris Johnson was on the pandemic in the run up to the first lockdown after questions over whether the Government was too slow to impose the first lockdown.
Former Health Secretary Matt Hancock's leaked WhatsApps have already shown how key decisions were discussed using the messaging app.
It comes as the Inquiry chair is expected to make her first public remarks since the row exploded on Tuesday ahead of the first public hearings of the Covid Inquiry next week.
Mr Abrahamson, who previously represented families during the Hillsborough Inquiry, warned Baroness Hallett could be forced to quit if she doesn't get the evidence she needs.
"This is a power struggle. It's a power struggle between politicians and the inquiry they set up," Mr Abrahamson told the press conference hosted by the Trades Union Congress (TUC).
"I hope and expect that the inquiry will win in that battle. There is a concern if they don't win, the chair having said quite clearly that she needs to see this material in order to have a proper inquiry.
"If the court says to her, 'Well, you can't see it,' then it seems to me the only logical response for the chair is to resign because she can't properly do her job.
"So I see this as an existential struggle between politics and the inquiry."
Mr Abrahamson said he was not calling for the chair to quit, but said losing the legal battle could mean it was her only option.
He said: "I'm not saying she should resign. I'm saying I expect that that might be the result because it's a matter of logic that the chair is saying 'I need this material to do my job properly'. That's why she's asking for the disclosure.
"So if you don't get the material to do your job properly, your choices are to do your job improperly or to say I can't work under those circumstances, and it seems to me logically the conclusion is that you should resign."
TUC General Secretary Paul Nowak said the Government can't act as "judge and jury" over the inquiry as ministers mount a desperate bid to block Baroness Hallett's demand for unredacted WhatsApps and documents.
He said: "The fact that the Prime Minister is prepared to spend taxpayers' money to try and block the handover of evidence is deeply worrying.
"The very least we deserve is transparency from our decision makers. No one should be allowed to hide from scrutiny.
"So today, I would issue a challenge to the Prime Minister: Treat this public inquiry with the respect it deserves and stop trying to undermine it."
He said the Government's "political games" should not distract from the critical work of the inquiry.
Lobby Akinnola, whose dad Femi, 60, died of Covid in April 2020, said Rishi Sunak must decide if he wants to be "antagonistic" towards the inquiry.
He said: "I think my response to the request for a judicial review would be to ask if there's an understanding of what the purpose of this inquiry is and whether or not the Prime Minister and the Cabinet want to be contributors to that or be antagonistic towards it.
"Because like we've said today, almost a quarter of a million people have died in this country, and we're trying to figure out how we make sure that doesn't happen again."
He added: "I understand a request for a right to privacy days, obviously an important way to maintain but also urge the Prime Minister to consider whether letting the inquiry see some text messages is significantly a significant inconvenience when you hold the lives of the nation in your hands."
The inquiry will begin by examining UK preparedness for a pandemic, before moving onto issues like Government decision making, vaccines and the care sector.
Module 1, which starts taking public evidence next week, will focus on the run up to the pandemic - with austerity PM David Cameron, the-then Chancellor George Osborne, and former Health Secretary Jeremy Hunt on the witness list.
Tonight, MPs lined up to criticise the Government's decision to go to court with the inquiry it established.
Tory former Justice Secretary Sir Robert Buckland urged ministers to rethink the decision and said judges would be reluctant to "second guess" the inquiry chair.
Tory grandee Sir Edward Leigh said: "Let everything hang out and just cooperate with the inquiry. "Let them have what they want and just get to the truth."
Cabinet Office Minister Jeremy Quin vowed the Government would "do absolutely nothing that we believe impedes the vital work of the inquiry".
He said the Government continued to cooperate with the Inquiry but needed legal guidance on a "narrow and technical point of law".
Mr Quin said: "Whereas it is entirely right that any material in any way related to Covid is available to the inquiry, we believe that there is value to challenge and debate in Government being unclouded by the knowledge that other discussions could be disclosed regardless of their relevance to any future inquiry."
He said the judicial review is likely to be heard “on or shortly after 30 June” but dodged questions on how much the legal battle will cost the taxpayer.
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