The Nagasaki Labor Bureau has recently warned three local staff agencies for illegally dispatching more than 500 plumbing workers to the crisis-hit Fukushima No. 1 nuclear plant last year, it was learned Saturday.

Labor laws ban "multiple dispatches," in which hired people end up working at places never mentioned in their initial contracts.

The three firms, Daiwa Engineering Service, Sowa Kogyo and Aguresu, all based in Nagasaki Prefecture, were involved in the illegal practice from July to August, the labor office said, adding that some workers also were paid less than they were promised.

The labor office has ordered the three firms to improve their business practices.

The case surfaced after a whistle-blower tipped off the labor office to the illegal dispatches.

Tokyo Electric Power Co., operator of the stricken plant, is struggling to secure the number of recruits needed to contain the crisis. The cumulative amount of radiation per employee is regulated by the government, and Tepco has been running out of workers.