A judge says Fox News has a “credibility problem” as it prepares for a defamation trial, after the company disclosed for the first time in nearly two years of litigation that Rupert Murdoch was an officer of the company.

On Monday, Fox News and its parent company Fox Corp head to trial over Fox’s coverage of false election-rigging claims. Mr Murdoch, chairman of Fox Corp, is expected to testify in the $US1.6 billion ($2.4 billion) trial.

Dominion Voting Systems alleges Fox damaged its business by knowingly and repeatedly airing false claims that Dominion machines were used to flip the 2020 US presidential election against former president Donald Trump, a Republican, and in favour of Democrat Joe Biden, who won.

Fox News had disclaimed that Mr Murdoch was a company officer, which shaped how Dominion litigated the case, according to Dominion lawyer Justin NelsonNelson. As a Fox News officer, Mr Murdoch would likely have been subject to more probing discovery by Dominion.

“It is very troubling, but this is where we are,” Mr Nelson said, adding that Dominion learned of Mr Murdoch’s role only on Sunday.

It received its first document with Mr Murdoch’s title of Fox News executive chairman on Tuesday.

A Fox lawyer told the hearing in Wilmington, Delaware, that Mr Murdoch disclosed the title in a February deposition and he called the title “honorific”.

A spokeswoman for Fox News did not immediately respond to a request for comment.

Superior Court Judge Eric Davis called the delayed disclosure “bizarre” and chided Fox lawyers for having made representations that Mr Murdoch wasn’t an officer of Fox News, only to reverse on the eve of trial.

“I’m not very happy right now,” Judge Davis said.

“You have a credibility problem.”

Dominion said depositions by Mr Murdoch and others, as well as troves of internal Fox communications, proved top network personnel knew the election-rigging claims were false but aired them anyway to appease Fox’s conservative viewers.

Determining who knew what when and how much power they had over Fox coverage is likely to be a critical task for jurors ahead of opening statements on Monday.

Dominion’s Mr Nelson said at Tuesday’s hearing the company was still trying to determine what action, if any, it would ask of the judge for the delayed disclosure.

– AAP