Andy Hall faces civil and criminal lawsuits submitted by Thai fruit processor Natural Fruit, a major supplier to the European drink market, in response to accusations of forced and child labour, unlawfully low wages and long hours.
Hall, who made the allegations in a report last year for Finnish rights watchdog Finnwatch, is due in court in Bangkok on September 2 on a defamation charge linked to an interview he gave to the Al Jazeera television network about the case.
If convicted in this first trial he could face one year in prison. More serious charges under the computer crime act -- which carries up to seven years in jail for each count -- are due to be heard later in September
Hall denounced the charges as "judicial harassment", saying he wanted his case to raise awareness of the "widespread and systematic" exploitation of migrants in Thailand.
"This abuse extends to many export markets well beyond fishing, seafood and pineapples to poultry, fruit and veg, rubber and even more now retail, food and beverage and tourism," he said in emailed comments to AFP.
Migrant workers, particularly from Myanmar and Cambodia, help keep major Thai industries from seafood to construction afloat, but they often lack official work permits and are paid below the minimum wage.
Thailand's junta rulers triggered an exodus of Cambodian workers following the May coup with a threat to arrest and deport illegal labourers, although thousands have since returned.
In the days after the mass departure the junta, formally known as the National Council for Peace and Order (NCPO), stressed the importance of migrant workers to Thailand's economy and said it will simplify the registration process for an official work permit.
The case against Hall, whose passport has been confiscated by Thailand pending his trial, has triggered international concern.
On Wednesday United Nordic, an alliance of food companies from northern Europe, said the case could further damage the Thai food industry.
"We consider the right way to handle such criticism to be presenting facts and engaging in constructive dialogue with all parties involved," United Nordic CEO Steinar Halvorsen told AFP.
A Natural Fruit factory in southern Thailand was investigated for a Finnwatch report co-authored by Hall, called "Cheap Has a High Price", because it produced pineapple concentrate for Finnish supermarkets' private label products, according to the watchdog.
In a statement Finnwatch called on Thailand, the world's largest pineapple producer, to change its approach "instead of issuing threats and exploiting workers".
Natural Fruit is also seeking $10 million through a civil suit.
Earlier this month nearly 100 international and national labour and human rights groups and NGOs sent a joint letter to members of the Thai Pineapple Industry Association calling on them to urge an end to the lawsuits by its member Natural Fruit.