Iwanaga Yoshiaki, Nishiyama Kazuhiro and Misato Foods joint stock companies were found breaching contracts they signed with interns, the Japanese Ministry of Health, Labor and Welfare’s immigration services agency said.

Iwanaga Yoshiaki made them work overtime and charged them higher rents than mentioned in the contract. Nishiyama Kazuhiro made them work overtime without paying for it. Misato Foods admitted it lacks the equipment mentioned in the contract that would allow interns to learn the job.

The agency did not say for how long the three have been blacklisted. 

Japanese authorities also issued warnings to two other firms, Sawa Joint Stock Company and Hitachi Co. Ltd, for failing to provide training programs that match contract terms.

The number of Vietnamese in Japan jumped by 26.1 percent last year to 330,835, accounting for 8 percent of all foreign nationals to become the third largest minority group behind Chinese and South Koreans, the Japan Times reported in March.

The growing presence of Japanese companies in Vietnam has sparked interest among younger Vietnamese in joining technical training programs in Japan, the report said.

According to the Japanese Immigration Department, Vietnamese surpassed Chinese as the largest group of technical trainees in the country last year after increasing by 30 percent to 164,499.