Former US president Donald Trump has urged Rupert Murdoch to embrace his false allegations of voter fraud if the media mogul testifies in the Fox News defamation trial.

With the $US1.6 billion ($2.4 billion) trial due to get underway in a Delaware courtroom within hours, Mr Trump took to his Truth Social platform on Monday (US time) with instructions for the Australian-born Fox boss.

“Fox News is in big trouble if they do not expose the truth on cheating in the 2020 election,” he wrote.

“They should do what’s right for America. When Rupert Murdoch says that there was no cheating in light of the massive proof that was there, it is ridiculous and very harmful to the fox case. Perhaps he should say that ‘he just didn’t know’, but that is hard to believe.

“Rupert, just tell the truth and good things will happen. The election of 2020 was rigged and stollen … you know it, and so does everyone else!”

Mr Trump, who was once a wild fan of Fox News, has grown increasingly critical of the network in recent months. He is particularly sensitive to its coverage of Florida governor Ron deSantis, seen as a key rival for the 2024 Republican presidential nomination.

“Back up those patriots at Fox, instead of throwing them under the bus,” Mr Trump urged Mr Murdoch.

The trial in the defamation lawsuit by Dominion Voting Systems against Fox Corp and Fox News will proceed with jury selection later on Tuesday (AEST).

The case, which stems from Fox’s airing of claims that Dominion’s machines were used to rig the 2020 US presidential election in favour of Democrat Joe Biden over Mr Trump, was due to have started on Monday, before Superior Court Judge Eric Davis.

But Judge Davis delayed it a day, without providing a reason. A source familiar with the matter told Reuters on Sunday that Fox had been pursuing a possible settlement.

• Fox News defamation trial about to begin: Here’s what to expect

Mr Trump’s post on Truth Social.

The Washington Post and the Wall Street Journal also reported that Fox was pursuing settlement talks, citing sources.

However, Judge Davis said later that it would go ahead, with no hint of a settlement being reached.

After jury selection, which began last week, is completed on Tuesday, opening statements to the 12-member panel will then be scheduled to follow, he said.

The trial is one of the most closely watched US defamation cases in years, involving a leading cable news outlet with numerous conservative commentators.

The primary question for jurors is whether Fox knowingly spread false information or recklessly disregarded the truth, the standard of “actual malice” that Dominion must show to prevail in a defamation case.

Mr Murdoch, the chair of Fox Corp, is expected to testify, along with a parade of Fox executives and on-air hosts including Tucker Carlson, Sean Hannity and Jeanine Pirro.

The trial is considered a test of whether Fox’s coverage crossed the line between ethical journalism and the pursuit of ratings, as Dominion alleges and Fox denies.

Fox has said the $US1.6 billion in damages being sought by Dominion is unrealistic and based on flawed economic modelling.

An expert report commissioned by Dominion attributed scores of lost contracts to Fox’s coverage although much of the report remains under seal.

Fox Corp reported nearly $US14 billion in annual revenue last year.

Fox claimed in filing on Sunday that Dominion had agreed to knock off more than $US500 million of its damages claim.

Fox’s filing cited a Friday email from a Dominion lawyer saying that Dominion would not pursue its lost profit claim at the trial.

A Dominion representative disputed that characterisation on Monday, saying in an email that the company’s damages have not changed.

Dominion has said Fox’s conduct was damaging to US democracy and that the network must be held accountable. In reply, Fox said last Friday that Dominion’s lawsuit was a threat to press freedom.

Also last week, Judge Davis sanctioned Fox News, handing Dominion a fresh chance to gather evidence after Fox withheld records until the eve of the trial.

The evidence to be presented in the trial includes recordings of Rudy Giuliani, Mr Trump’s lawyer, saying in pre-taped Fox appearances that he did not have any evidence to back up the allegations of election rigging by Dominion.

The recordings were made by a former Fox producer in her separate lawsuit against the network.

Judge Davis said he would also very likely tap an outside investigator to probe Fox’s late disclosure of the evidence and take whatever steps necessary to remedy the situation, which he described as troubling.

-with AAP