Employers sought registration for 772,270 undocumented migrant workers over the last two weeks and 58% of them were from Myanmar, according to the Labour Ministry.
Spokesman Ananchai Uthaipatanacheep said on Tuesday the migrant workers were reported by 193,918 employers, about 70% of them individuals and the rest juristic persons.
With Myanmar nationals making up the majority, Cambodians were the second most numerous group (28.86%) and then Lao nationals (12.67%).
Their most common occupations were in the construction sector with 181,772 workers, followed by 170,854 workers in agriculture, 70,384 in the food and beverage sector, 58,914 in services, and 51,512 in domestic service.
The highest number of reported illegal migrant workers was in Bangkok (152,903), followed by Chon Buri (36,734), Samut Prakan (33,045), Pathum Thani (28,521) and Rayong (28,320).
The provinces with the fewest were Yasothon (104), Uttradit (157), Roi Et (161), Amnat Charoen (167) and Maha Sarakham (184).
Employers who failed to report their illegal migrant workers to authorities during the July 24 - Aug 7 registration period have missed the final chance of leniency offered by the government, said Mr Ananchai, who is also a labour inspector-general.
Their illegal migrant workers would have to be repatriated. Their employers would from then on have to recruit only legal migrant workers, or be penalised.
The new executive decree on the illegal employment of foreign workers provides for fines of up to 800,000 baht per illegal migrant worker. The penalty was postponed until the end of this year, pending processes to legalise the newly registered workers.
After the two-week application period, the process of verifying the employment of the newly registered migrant workers started on Tuesday.
Verification would be done by interviews with employers and workers. Afterwards the workers' nationalities would have to be verified by authorities from their homeland before work permits were issued.
Myanmar authorities will have nationality verification services in Bangkok, Chiang Mai, Chiang Rai, Ranong, Samut Prakan, Samut Sakhon, Songkhla and Tak provinces, he said.
Cambodian verification services will be in Bangkok, Rayong and Songkhla.
Laos had yet to confirm if it would provide the service in Thailand. Pending that, Lao workers would have to seek citizenship verification in their homeland, Mr Ananchai said.