A Thai court has dismissed a criminal defamation case against a British human rights activist who investigated alleged abuses at a Thai fruit processing factory.

Andy Hall still faces charges in three other cases filed by the company Natural Fruit. The next trial begins tomorrow.

His troubles began after he helped write a report last year for the Finland-based watchdog Finnwatch on what it called poor labour conditions in seafood and pineapple export companies in Thailand, including Natural Fruit. The company denies it.

The first case against him related to defamation charges for an interview he gave to Qatar-based Al-Jazeera television.

The court dismissed the case today because the interview took place in Burma, and Thailand's attorney general office was not involved into the investigation as required by law.

The report investigated a factory owned by Natural Fruit that employed hundreds of migrants from neighbouring Burma and alleged the company illegally confiscated passports, paid below minimum wage and overworked staff in sweltering conditions so hot that heat strokes were common.

Sonja Vartiala, the executive director of Finnwatch, said she was "relieved and glad that justice has prevailed".

But she warned that Mr Hall still faces up to seven years in jail and fines totalling £8.7 million if found guilty in the remaining cases. He is currently out on bail in Thailand.

"Finnwatch demands Natural Fruit now drop all the charges against Andy Hall. Instead of allowing companies to bring human rights activists to court, Thailand needs to prosecute companies like Natural Fruit, who are violating labour rights," she said.

Mr Hall has worked in Thailand for years and is an outspoken activist on migrant issues.

Millions of impoverished migrants, largely from Burma and Cambodia, work in Thailand. Some do not have legal papers, and many work low-skilled jobs for long hours at pay below their Thai counterparts. They typically lack health and social security benefits.

The trial comes after the United States demoted Thailand earlier this year to the lowest level in its annual rankings of governments' anti-human trafficking efforts, principally over its failures to do enough to stop abusive practices in the Thai seafood industry.