TOKYO (Reuters) - Japanese authorities may have underestimated by 20 percent the radiation doses workers got in the initial phase of the Fukushima nuclear power plant disaster, a Japanese newspaper reported on Saturday, citing a U.N. panel.

A big earthquake and tsunami in March 2011 damaged the power station north of Tokyo, operated by Tokyo Electric Power Co, causing three partial reactor meltdowns. The company has struggled to contain leaking radiation since then.

The U.N. Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) raised doubts about the dose estimates of the government and Tokyo Electric, or Tepco, in a summary of a report on October 12, according to the Asahi Shimbun.

The U.N. committee analyzed radiation doses in 25,000 people who worked at the plant 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 short half-lives of 2 hours and 20 hours, respectively.

The Asahi Shimbun said if the U.N. 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.

A spokesman for Tepco was not immediately available for comment.

(Reporting by Lisa Twaronite; Editing by Robert Birsel)