The reports cited a Justice Ministry investigation, but the ministry declined immediate comment.

The reports are likely to revive criticism of the foreign trainee programme, which has been accused of placing workers in substandard conditions and jobs that provide few opportunities for learning.

The misconduct was uncovered in a probe by the justice ministry conducted after a Vietnamese trainee was found in March to have participated in cleanup work in Fukushima, public broadcaster NHK said.

A powerful earthquake in March 2011 spawned a huge tsunami that led to meltdowns at the Fukushima nuclear plant, causing the world's worst such accident since Chernobyl in 1986.

The justice ministry said after the March discovery that decontamination work was not appropriate for foreign trainees.

It was not immediately clear how many trainees were forced to take part in the decontamination work. Local media said one of the four companies has been slapped with a five-year ban on accepting new foreign trainees.

The ministry has finished its probe into 182 construction companies that hire foreign trainees, and will look into another 820 firms by the end of September, local media said.

Japan has been accepting foreign trainees under the government programme since 1993 and there were just over 250,000 in the country in late 2017.

But critics say the trainees often face poor work conditions include excessive hours and harassment.

The number of foreign trainees who ran away from their employers jumped from 2,005 in 2012 to 7,089 in 2017, according to the ministry survey. Many cited low pay as the main reason for running away.

The report comes as Japan's government moves to bring more foreign workers into the country to tackle a labour shortage caused by the country's ageing, shrinking population.

The government in June said it wanted to create a new visa status to bring in foreign workers, with priority given to those looking for jobs in sectors such as agriculture that have been hardest hit by the labour shortage.

The workers would be able to stay for up to five years, but would not be allowed to bring their family members.

The government put the number of foreign workers in Japan in 2017 at 1.28 million people.

But more than 450,000 of those are foreign spouses of Japanese citizens, ethnic Koreans long settled in Japan, or foreigners of Japanese descent, rather than workers coming to Japan simply for jobs.

Another nearly 300,000 are students.