A construction worker walks near water tanks at the Fukushima Dai-ichi nuclear plant on June 12 2013

The operator of Japan’s Fukushima Daiichi nuclear power station has come under fire for its slow response to evidence that contaminated groundwater has been escaping from the tsunami-crippled facility and flowing into the ocean.

Government officials and local fishermen criticised Tokyo Electric Power on Tuesday after it admitted the existence of the leak for the first time, weeks after regulators spotted the problem and days after the utility’s own analysis confirmed it.

“The data disclosure was very slow, and that is extremely regrettable,” Toshimitsu Motegi, industry minister, told reporters.

Yoshihide Suga, the chief cabinet secretary, also said Tepco should have responded more quickly.

Tepco confirmed on Monday what experts and regulators had suspected for some time: that it is failing to prevent the emission of toxic water that has collected under the plant since it suffered multiple core meltdowns in the disaster in March 2011.

The utility’s investigation of the problem appears to have accelerated as a result of pressure from the Nuclear Regulation Authority, the new and more independent safety regulator that was set up after the accident. Shunichi Tanaka, the head of the NRA, said this month he suspected that tainted groundwater was leaking into sea.

Tepco first detected a rise in the level of radioactive material – including caesium, tritium and strontium – in the plant’s groundwater in May. It initially said there was no evidence that the contaminated water was leaking into the sea, but agreed to conduct more tests after the regulator went public with its suspicions.

Anger at Tepco was compounded by reports that the utility had completed its analysis last week, but waited until Monday to report the results formally to the NRA and the public. Some posters in online forums suggested the utility had waited until after elections for the upper house of parliament on Sunday.

The issue could influence the debate over whether to restart other nuclear facilities that have been idled since the Fukushima accident, a policy that is supported by the government of Shinzo Abe, the prime minister, but which surveys suggest is opposed by about 55 per cent of citizens.

The leak also highlights the precarious nature of the clean-up work being done at Fukushima, where a full decommissioning, including the removal of radioactive fuel and other dangerous materials, is expected to take decades.

Tetsu Nozaki, the head of Fukushima prefecture’s fishermen’s association, said the leak showed that the plant still presented a danger well after the damaged reactors were brought to a relatively stable state of “cold shutdown” in December 2011. “I want to ask the government to rescind its declaration that the crisis is over,” he was quoted by Japanese media as saying.

Tepco argues the leak poses little if any danger to humans or the environment because the contaminants have been kept within a small radius around the facility, inside an area where it has erected silt barriers and other defences.

Tepco this month detected tritium, an isotope of hydrogen, in the sea near the reactors’ water intakes in amounts of up to 2,300 becquerels per litre. That is a relatively low concentration but still 20 times the level found in April. In wells inside the grounds, levels as high as 600,000 becquerels per litre – 10 times the official safety threshold – have been found.

Clean-up crews have been injecting coolant water into the cores of the stricken reactors since the accident, and a massive network of pipes, storage tanks and filtration systems has been erected to recycle it and prevent leakage.

But the damage that the facility sustained in the earthquake that triggered the tsunami, as well as explosions during the first days of the crisis, has made it all but impossible to seal it completely.

It is not the first time that Tepco has been faulted for disclosure problems. Only last week it increased by a factor of 10 its estimate of the number of plant workers who have been exposed to enough radiation to put them at increased risk of thyroid cancer, after experts questioned the methodology behind its previous estimate.

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