Andy Hall can face up to seven years in prison if found guilty of defamation over his research on tinned fruit industry.
Tens of thousands of workers have fled Thailand, officials said, after new labour regulations adopted by the military government sparked fear and panic among the migrant community.
About 60,000 workers left between June 23 and 28, and the number has risen since, an immigration official said on Monday.
“They were of all nationalities, but the biggest group was from Myanmar,” Deputy Commissioner Pornchai Kuntee told Reuters news agency. “They are probably very scared.”
A new decree includes fines that can range up to 800,000 baht ($23,557) for employers who hire unregistered foreign workers without permits.
Following initial news of the exodus, authorities promised a 120-day delay in enforcing the new regulations.
During that period, there will be no arrests or crackdown on illegal workers except “for those who violate human trafficking laws,” Deputy Prime Minister Wissanu Krea-ngam told reporters on Friday.
Millions of workers from poor neighbours, such as Cambodia and Myanmar, form the backbone of Thailand’s manual labour force, with industries such as the multibillion-dollar seafood business heavily reliant on foreign workers.
Since taking power in a 2014 coup, Thailand’s military government has attained varying degrees of success in campaigns to regulate the foreign workforce, spurred on partly by media reports that unregulated workers faced exploitation by employers.
Geta Devi, 28, a Myanmar worker based in the Thai capital of Bangkok, said some of her friends panicked over the decree and “went back to Myanmar”.
Since last week, up to 500 Cambodian migrant workers have returned home, said Chin Piseth, deputy chief of the Thai-Cambodia border relations office of the Cambodian army.
“According to reports I received, between 400 and 500 were deported,” he told Reuters news agency.
Andy Hall, a British specialist in migrant workers’ rights, said the mass movement leaves undocumented workers vulnerable.
“It’s clear to me tens of thousands of migrants only move like this after instigation,” Hall said.
Despite the threat of punishment, “corrupt officials” would try to seek bribes, he said, adding that “mass profit is to be made in a short time from the panic and commotion.”
Police trying to extort money from employers or migrant workers face punishment, Thai police chief Chaktip Chaijinda said on Friday, in an effort to discourage such exploitation.
Last month, the US kept Thailand on a trafficking watch list, saying it did not meet the minimum standards to end human trafficking.
Thailand defended its efforts to stop trafficking and urged US officials to visit and gauge its campaign.
More than three million migrants work in Thailand, the International Organization for Migration says, but rights groups put the figure higher.
In 2014, at least 250,000 migrants workers fled Thailand over what rights organisations called “cruel, inhumane and degrading treatment”.