The Japanese energy ministry says radioactive water from the crippled Fukushima nuclear plant is leaking into the sea at the rate of 300,000 litres a day, and has probably been doing so since the plant was damaged by the March 2011 earthquake and tsunami.
The company that runs the plant, Tepco, says it is not clear how much water has been getting into the Pacific Ocean or how much of it is contaminated.
Prime Minister Shinzo Abe says containing the leak is an urgent issue, the BBC reports, and he's promising more than $500 million of government money to address the situation.
Japan's nuclear watchdog, the Nuclear Regulatory Authority, says the build-up of radioactive groundwater has created a new emergency, as a barrier built to contain the water has already been breached and the amount of contaminated water seeping into the sea could accelerate rapidly.
Tepco admitted for the first time last month that radioactive groundwater had breached the barrier but said it was taking steps to prevent it.
However, the head of a Nuclear Regulatory Authority taskforce, Shinji Kinjo, has told the Reuters news agency that Tepco's "sense of crisis is weak" and "you can't just leave it up to Tepco alone"
If the underground barrier is breached, the watchdog warns, the water could start to seep through shallower areas of earth, and once it reaches the surface, Mr Kinjo says, it could start to flow "extremely fast".
Contaminated water could rise to the ground's surface within three weeks, the Asahi newspaper predicted last Saturday.
The contaminated water is thought to have come from the 400 tonnes of groundwater pumped into the plant every day to cool the reactors.