Six people on a foreign trainee program in Japan engaged in construction work at the disaster-stricken Fukushima Daiichi nuclear plant despite the plant operator's ban on program participants working at the complex, officials said Tuesday.

The case is the latest in a string of inappropriate practices involving foreign trainees under the government's Technical Intern Training Program, often criticized as a cover to import cheap labor.

Tokyo Electric Power Company Holdings Inc. had said in February last year that it would not have foreign trainees work at the plant, which was crippled by the 2011 quake and tsunami disaster, even though workers in some parts of the plant are not required to wear protective gear or dosimeters.

The six people were hired by one of Tepco's subcontractors. "We deviated from our independent rules on employment. We will make our subcontractors thoroughly check the terms of their contracts," a Tepco official said.

According to the utility, the foreign trainees took part in groundwork at the plant starting in November last year outside the areas where protective measures against radiation are needed. The trainees had not received any training on how to protect themselves from radiation.

Japan introduced the intern program in 1993 with the aim of transferring skills to developing countries. But the scheme, applicable to agriculture and manufacturing among other sectors, has drawn criticism as a number of harsh and exploitative cases have been reported.

Earlier in the year, several Vietnamese trainees hired by construction companies in Koriyama, Fukushima Prefecture and Morioka, Iwate Prefecture, were found to have engaged in radioactive contamination cleanup work in Fukushima.

The Justice Ministry was conducting a probe into companies hiring trainees, saying decontamination work does not fit the purpose of the trainee program.