Tokyo (CNN) -- Japan's Ministry of Health, Labor and Welfare is investigating a report that workers at the damaged Fukushima Daiichi nuclear power plant were told to use lead covers in order to hide unsafe radiation levels, an official said.
The alleged incident happened December 1, nine months after a major earthquake and tsunami ravaged northern Japan and damaged the plant.
"We'll firmly deal with the matter once the practice is confirmed to constitute a violation of any law," said the ministry official, who could not be named in line with policy.
An official with the plant's operator, TEPCO, said the company received a report of the alleged incident Thursday from subcontractor Tokyo Energy & Systems. The report said a second subcontractor, Build-Up, created the lead covers and ordered workers to use them over their dosimeters, pocket-size devices used to detect high radiation levels.
The TEPCO official could also not be named in line with policy.
Tokyo Energy & Systems said in its report that the workers never used the covers, the TEPCO official said. Japan's Asahi Shimbun newspaper, however, reported Saturday that while some workers refused the orders to use the lead covers, nine others did use them for several hours.
The newspaper's report cited plant workers, who described the lead covers as fitting snugly over the dosimeters inside the breast pockets of the workers' protection suits.
TEPCO told CNN it ordered Tokyo Energy & Systems Inc. to conduct an investigation and is awaiting a reply.
CNN's Junko Ogura contributed to this report.