One of Thailand’s most prominent union leaders is facing three years in prison for his role in organising a railway safety campaign, in a case described as the biggest attack on organised labour in the country in decades.

Rights advocates say the case involving Sawit Kaewvarn, president of the State Railway Union of Thailand, will have a chilling effect on unions and threatens to further weaken workers’ rights in the country.

Sawit, who is also head of the State Enterprises Workers’ Relations Confederation, the largest body of trade unions in Thailand, was convicted of omission of duties and sentenced to three years in prison last October. Twelve other national and local union leaders received the same sentence.

The group has been freed on bail and have appealed against the verdict, with a ruling expected soon.