Japan's nuclear watchdog has said the crippled Fukushima nuclear plant is facing a new "emergency" caused by a build-up of radioactive groundwater.
A barrier built to contain the water has already been breached, the Nuclear Regulatory Authority warned.
This means the amount of contaminated water seeping into the Pacific Ocean could accelerate rapidly, it said.
There has been spate of water leaks and power failures at the plant, devastated by the 2011 earthquake and tsunami.
Its operator, Tokyo Electric Power Company (Tepco), has been criticised heavily for its lack of transparency over the leaks.
'Weak sense of crisis'
Tepco admitted for the first time last month that radioactive groundwater had breached an underground barrier and been leaking into the sea, but said it was taking steps to prevent it.
However, the head of a Nuclear Regulatory Authority task force, Shinji Kinjo, told the Reuters news agency on Monday that the countermeasures were only a temporary solution.
Tepco's "sense of crisis is weak," Mr Kinjo said. "This is why you can't just leave it up to Tepco alone"
"Right now, we have an emergency," he added.
If the underground barrier is breached, the watchdog warns, the water could start to seep through shallower areas of earth.
Once it reaches the surface, it could start to flow "extremely fast", says Mr Kinjo.
Contaminated water could rise to the ground's surface within three weeks, the Asahi newspaper predicted on 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.
Tepco 'in trouble'
Tepco admitted on Friday that a cumulative 20 trillion to 40 trillion becquerels of radioactive tritium may have leaked into the sea since the disaster.
It has been clear for months now that the operators of the Fukushima plant are in deep trouble, says the BBC's Rupert Wingfield-Hayes.
The only course of action, he continues, is to pump water out. But this has to be stored, and more than 1,000 giant holding tanks surrounding the plant are nearly all full, he adds.
Tepco said on Monday it plans to start pumping out a further 100 tonnes of groundwater a day.