Japan's labour minister on Tuesday ordered an investigation into claims that subcontractors at the crippled Fukushima nuclear plant told workers to lie about their radiation exposure.

Yoko Komiyama told a press briefing in Tokyo that she had launched a wide-ranging probe, including checks on a firm at the centre of allegations which have appeared in Japanese media in recent days.

"This is an issue that shakes the foundation of the management of workers' radiation exposure," said the minister for health, labour and welfare.

"We will deal with it in a strict manner if any laws were broken. (If true) this is extremely regrettable," she added.

An executive at construction firm Build-Up in December told about 10 workers to cover their dosimeters -- used to measure cumulative radiation exposure -- with lead casings when working in areas of high radiation, the Asahi Shimbun newspaper and other media said.

The move was aimed at under-reporting employees' exposure to radiation so the firm could continue working at the site of the worst nuclear disaster in a generation, the media reports said.

A 9.0-magnitude earthquake and tsunami on March 11, 2011 crippled cooling equipment at the Fukushima Daiichi atomic plant, triggering meltdowns that spewed radioactivity and forced tens of thousands of residents to flee.

Several workers at Build-Up reportedly said that their on-site supervisor told them he had used a lead casing and urged them to do the same, otherwise they would quickly reach their legally permissible annual radiation exposure.

"Unless we hide it with lead, exposure will max out and we cannot work," the executive was heard saying in a covert audio recording, the Asahi reported.

Some workers refused and left the company, the newspaper said.

The workers were hired for about four months to insulate pipes at a water treatment facility, Kyodo News has said.