Japanese authorities may have underestimated by 20 percent the radiation doses workers got in the initial phase of the Fukushima nuclear disaster, a UN panel has said.

A big earthquake and tsunami in March 2011 damaged the power station 150 miles north of Tokyo, causing three partial reactor meltdowns.

The operator, Tepco, has struggled to contain leaking radiation ever since.

The UN Scientific Committee on the Effects of Atomic Radiation raised doubts about the dose estimates of the government and Tepco, in a summary of a report on October 12, according to the Asahi Shimbun newspaper on Saturday.

The UN committee analysed radiation doses in 25,000 people who worked at the station on or before October 2012, using data provided by the government, Tepco and others, the newspaper said.

It determined that the tests used on workers did not take into account some types of radiation.

In particular, workers were tested for thyroid gland doses from radioactive iodine after a significant delay, through procedures that failed to account for iodine-132 and iodine-133, which have half-lives of two hours and 20 hours respectively.

The Asahi Shimbun said if the UN panel's assessment was accurate, more workers would be eligible for free health checks.

It did not give any detail of the implications on the health of the workers.

Increased radiation exposure has been linked to greater rates of cancer and thyroid disorders.