Media playback is unsupported on your device Media caption The arrest of Mr Hall throws a spotlight onto labour rights in Thailand, as Jonathan Head explains

A British activist who campaigns for better working conditions has gone on trial in Thailand for defamation.

The alleged offence is one of a series of charges relating to a report by a Finnish watchdog group last year which Andy Hall helped to write.

He was charged after a complaint by Thailand's Natural Fruit Company, one of the companies he wrote about.

Mr Hall stands by his report and has told reporters that the charges against him amount to "judicial harassment".

Defamation is a criminal offence in Thailand and Mr Hall could be jailed for a year if found guilty.

But Mr Hall is also facing charges under the computer crime act, which can result in seven years in jail. On top of that the company is seeking $10m (£6m) in a civil suit.

The report he helped to write, for the group Finnwatch, said Natural Fruit Company, the country's largest producer of pineapples, mistreated its workers - a claim the company denies.

Mr Hall told reporters before entering the court: "I don't believe there's any evidence against me to show that what I did was malicious or in some way against the company. I did it for the benefit of the workers."

But Natural Fruit's owner, Wirat Piyapornpaiboon, told the AFP news agency: "The report caused damage to me and my company. Any accusations were not true... If true, why are there so many workers who want to work at my factory?"

Image copyright Jonathan Head Image caption Natural Fruit, owned by Wirat Piyapornpaiboon, is seeking $10m in a civil suit against Mr Hall

The BBC's Jonathan Head in Bangkok says a number of European retail and labour organisations have called on Natural Fruit to withdraw its charges and instead investigate Mr Hall's claims.

Thailand has grown to become one of the world's biggest food producers, but it has been criticised for the treatment of migrant workers.

The frequent use of the criminal defamation law in Thailand to silence critics has been condemned by human rights groups.

The US state department this year demoted Thailand on a list measuring how well a country combats human trafficking. It is currently in a tier for countries which are not making significant efforts to comply with US standards.

Thailand relies significantly on migrant workers from neighbouring Myanmar and Cambodia, but reports say that some do not have legal papers and many work long hours for lower pay than Thai workers.

Tens of thousands of Cambodian workers in Thailand fled home across the border in June, prompted by fears that the new military government would crack down on migrants.