Slow efforts to register illegal immigrants have sparked fears that companies may continue to exploit workers from poorer bordering countries.
Labour Department officials say there are between 700,000 and 1.2 million illegal immigrants working in Thailand, many paid salaries much lower than the minimum wage.
In an effort to curb exploitation and to fight a shortage of workers in labour-intensive industries, the Government has offered an amnesty which allows workers to register for work permits.
Providing applicants pass health checks - to stem diseases like elephantiasis, malaria, and tuberculosis - permits are valid for two years in the construction, fisheries and agriculture sectors.
But labour officials said many illegal immigrants who have long been in hiding still fear the authorities and believe the move will make it easier to send them home after their work permits expire.
Many workers in border provinces cannot speak Thai and are known to run at the sight of police or officials.