Highly radioactive water from Japan's crippled Fukushima nuclear plant is pouring out at a rate of 300 tonnes a day, officials said.
Prime Minister Shinzo Abe ordered the government to step in and help in the clean-up after the revelation, which amounted to an acknowledgement that plant operator Tokyo Electric Power Co (Tepco) has yet to come to grips with the scale of the catastrophe, two-and-a-half years after the plant was hit by a huge earthquake and tsunami.
Tepco only recently admitted water had leaked at all, but it appears the leak from the plant 130 miles northeast of Tokyo is enough to fill an Olympic swimming pool in a week. The water is spilling into the Pacific Ocean, but it was not immediately clear how much of a threat it poses.
Calling water containment at the Fukushima Daiichi station an "urgent issue," Abe ordered the government for the first time to get involved to help struggling Tepco handle the crisis.
As early as January this year, Tepco found fish contaminated with high levels of radiation inside a port at the plant. Local fishermen and independent researchers had already suspected a leak of radioactive water, but Tepco denied the claims.
Tetsu Nozaki, the chairman of the Fukushima fisheries federation said he had only heard of the latest estimates of the magnitude of the seepage from media reports.
Environmental group Greenpeace said Tepco had "anxiously hid the leaks" and urged Japan to seek international expertise.
"Greenpeace calls for the Japanese authorities to do all in their power to solve this situation, and that includes increased transparency...and getting international expertise in to help find solutions," Dr. Rianne Teule of Greenpeace International said in an e-mailed statement.
In the weeks after the disaster, the government allowed Tepco to dump tens of thousands of tonnes of contaminated water into the Pacific in an emergency move, but the escalation of the crisis raises the risk of an even longer and more expensive clean-up, already forecast to take more than 40 years and cost $11bn (£7bn).
The admission further dents the credibility of Tepco, criticised for its failure to prepare for the tsunami and earthquake, for a confused response to the disaster and for covering up shortcomings.
"We think that the volume of water (leaking into the Pacific) is about 300 tonnes a day," said Yushi Yoneyama, an official with the Minister of Economy, Trade and Industry, which oversees energy policy.
Tatsuya Shinkawa, a director in METI's Nuclear Accident Response Office, told reporters the government believed water had been leaking for two years, but Yoneyama told Reuters it was unclear how long the water had been leaking at the current rate.
Shinkawa described the water as "highly" contaminated.
The water is from the area between the crippled reactors and the ocean, where Tepco has sought to block the flow of contaminated water by chemically hardening the soil. Tetsu Nozaki, head of the Fukushima fisheries federation called for action to end the spillage.
"If the water was indeed leaking out at 300 tonnes a day for more than two years, the radiation readings should be far worse," Nozaki told Reuters. "Either way, we have asked Tepco to stop leaking contaminated water into the ocean."
"Rather than relying on Tokyo Electric, the government will take measures," Prime Minister Abe said after instructing METI Minister Toshimitsu Motegi to ensure Tepco takes appropriate action.
The prime minister stopped short of pledging funds to address the issue, but the ministry has requested a budget allocation, an official told Reuters.
The Nikkei newspaper said the funds would be used to freeze the soil to keep groundwater out of reactor buildings - a project estimated to cost up to 40 billion yen (£270m).
Tepco's handling of the clean-up has complicated Japan's efforts to restart its 50 nuclear power plants. All but two remain shut since the disaster because of safety concerns. That has made Japan dependent on expensive imported fuels.