LOS ANGELES (BLOOMBERG) - Billionaire Elon Musk will have to go to trial in December after a federal judge rebuffed his latest request to throw out a defamation lawsuit filed by a UK caver after Mr Musk referred to him as a "pedo guy".

US District Judge Stephen Wilson in Los Angeles on Monday (Oct 28) ruled that a jury will have to decide whether Mr Musk was negligent for failing to check that statements he made in his tweets and "off-the-record" e-mails were true.

The Tesla CEO's accusations, the judge said, were not germane to any public controversy involving Mr Vernon Unsworth, which might have entitled him to a free-speech defence.

The burden of proof for negligence is lower than that for actual malice, Mr Unsworth's lawyer Lin Wood said after the hearing.

Mr Unsworth will have to persuade the jury by a preponderance of evidence rather than clear and convincing evidence, which will make it easier for the panel to find Mr Musk liable for defamation, according to Mr Wood.

Whether or not Mr Musk acted with malice could still be used in seeking punitive damages, Mr Wood said.

"We look forward to the trial," Mr Alex Spiro, an attorney for Mr Musk, said in a statement.

"We understand that, while Musk has apologised, Unsworth would like to milk his 15 minutes of fame."

Mr Musk started his public spat with Mr Unsworth last year after the caver said in an interview on CNN that a mini submarine Mr Musk had sent to Thailand to help in a rescue of a group of boys trapped in a cave was a "PR stunt".

Mr Unsworth added that Mr Musk could "stick his submarine where it hurts".

Mr Musk responded by calling Mr Unsworth a "pedo guy" in a tweet and a "child rapist" in an e-mail to a BuzzFeed reporter.

Mr Wilson wasn't persuaded by arguments from Mr Musk's attorney, Mr Robert Schwartz, at Monday's hearing that any alleged paedophilia was somehow relevant to the question whether Mr Musk's submarine might have worked in the rescue or was public relations stunt.

"You're saying his being a paedophile is germane to the issue whether Mr Musk had sincere motivation", Mr Wilson said. "That seems a bit of a stretch."

WRITTEN RULING

The judge said he would issue a written ruling later. However, by throwing out Mr Musk's First Amendment defence that he was engaging with a public figure in a public controversy, Mr Unsworth doesn't have to prove Mr Musk acted with actual malice to win on his defamation claim.

The case took a new twist this month after court documents showed that Mr Musk, after his initial tweets in July of last year, had hired a private investigator to dig up dirt about Mr Unsworth. It turned out the detective was a convicted conman.

Mr Musk then sent an "off-the-record" e-mail to a BuzzFeed reporter in which he said Mr Unsworth had married a 12-year-old child bride in Thailand.

Mr Unsworth says that's not what the investigator told Mr Musk.

Mr Unsworth said in court filings that he met his Thai wife in London when she was 32 years old. The caver also scoffed at Mr Musk's defence that he didn't intend for his "off-the-record" e-mail to be published.

Mr Unsworth said Mr Musk "admitted that he wanted the information published whether true or false, and told the reporter that publication is 'up to you'."