(Reuters) — Tesla boss Elon Musk is set to go on trial on Tuesday after he described a British cave explorer who helped rescue a group of boys from a cave complex in Thailand as a pedophile on Twitter.

Vernon Unsworth, who played a leading role in the recovery of the 12 boys and their soccer coach from Tham Luang Nang Non cave in July 2018, has said Mr. Musk falsely labeled him a "pedo guy" on Twitter.

He says Mr. Musk, whose Twitter habits have long been under a microscope, should pay punitive and other damages for harming his reputation.

The trial before U.S. District Judge Stephen Wilson in Los Angeles is scheduled to last about five days. Mr. Musk is expected to testify in his own defense. He has apologized for the remark.

While the case does not involve Tesla, investors and regulators have expressed concerns about the accuracy of his tweets.

With 29.8 million followers, Mr. Musk's Twitter account is a major source of publicity for his Palo Alto, California-based electric car company, which does not advertise.

The Unsworth case is among the last issues hanging over Mr. Musk from a turbulent 2018 and early 2019, when he regularly clashed with Wall Street and short sellers as Tesla struggled with production problems.

The episode began after Mr. Musk offered a mini-submarine from his SpaceX rocket company to help with the cave rescue.

Mr. Unsworth told CNN on July 13, 2018, three days after the rescue was completed, that the offer was a "PR stunt" and that Mr. Musk could “stick his submarine where it hurts."

Two days later, Mr. Musk lashed out at Mr. Unsworth in a series of tweets, including one which called him a "pedo guy." Mr. Musk later apologized for that comment.

Mr. Unsworth has denied Mr. Musk's accusations.

To win the defamation case, Mr. Unsworth needs to show that Mr. Musk was negligent, which does not require an intent to defame.

He must prove the tweets were false, that Mr. Musk did not use reasonable care to determine if they were true, and that people reasonably understood them to mean he was a pedophile.

The case against Mr. Musk got a boost when Judge Wilson last month said Mr. Unsworth's sudden fame from the rescue did not make him a "public figure," meaning he did not need to show that Mr. Musk acted with "actual malice" when posting his tweets.

Lawyers for Musk have said the tweets were opinion, not statements of fact, and that Unsworth knew of no one who thought he was a pedophile based on them.

They have also said Unsworth sought to profit from his role in the rescue and provoked Musk's response by suggesting on CNN that Musk did not care about the lives of the trapped boys.