The operator of the crippled Fukushima nuclear plant on Monday admitted for the first time that radioactive groundwater has leaked out to sea, fuelling fears of ocean contamination.

The admission came the day after Japanese voters went to the polls in an election for the upper house, handing the largely pro-nuclear party of Prime Minister Shinzo Abe a handsome majority.

Earlier this month Tokyo Electric Power (TEPCO) said groundwater samples taken at the battered plant showed levels of possibly cancer-causing caesium-134 had shot up more than 110 times in a few days.

TEPCO did not know the exact reasons for the increased readings but had maintained the toxic groundwater was likely contained at the current location, largely by concrete foundations and steel sheets.

"But now we believe that contaminated water has flown out to the sea," a TEPCO spokesman said Monday.

However, the spokesman insisted that the impact of the radioactive water on the ocean would be limited.

"Seawater data have shown no abnormal rise in the levels of radioactivity."

Another company spokesman later told a news conference in Tokyo: "We sincerely apologise for worrying many people, especially people in Fukushima."

TEPCO, which is surviving with massive public funds, said that it would step up efforts to reduce underground water by consolidating soil near the harbour.

Radioactive substances released by the meltdowns of reactors at the plant in the aftermath of the huge tsunami of March 2011 have made their way into underground water, which usually flows out to sea.

Environment experts warn that such leakage may affect marine life and ultimately impact humans who eat sea creatures.

Tetsu Nozaki, chairman of Fukushima Prefectual Federation of Fisheries Co-operative Associations, voiced deep concern over the leakage.

"It was quite shocking," he told Japanese public broadcaster NHK. "(TEPCO's) explanation is totally different from the one in the past."

Fishing around Fukushima was halted and the government banned beef, milk, mushrooms and vegetables from being produced in surrounding areas.

TEPCO said earlier this year that a fish contaminated with radiation levels more than 2,500 times the legal limit had been caught at a port inside the Fukushima plant.

Last week, the firm also said around 2,000 people who have worked at the Fukushima nuclear plant face a heightened risk of thyroid cancer.

The figure is a 10-fold increase on TEPCO's previous estimate of the number of possible thyroid cancer victims and comes after the utility was told its figures were too conservative.

Tens of thousands of people were forced from their homes by the threat of radiation after the tsunami and Fukushima disaster, with some still unable to return.

Although the nuclear accident is not officially recorded as having directly killed anyone, the natural disaster claimed more than 18,000 lives.