Around 100 tonnes of highly radioactive water have leaked from a storage tank at Japan's Fukushima nuclear plant, operator Tokyo Electric (Tepco) says.
The toxic water may have overflowed after a valve was left open by mistake, Tepco said.
However the water was unlikely to have reached the ocean, the operator added.
The plant, which was damaged by an earthquake and tsunami in 2011, has faced multiple problems including leaks and power cuts since the disaster.
The latest leak is the most serious since August, when the plant leaked 300 tonnes of water, prompting Japan's nuclear agency to raise the incident's alert level.
The water from Wednesday's leak was radioactive, with a reading of 230 million becquerels per litre of radioactive isotopes, Tepco spokesman Masayuki Ono told reporters.
A becquerel is a unit used to measure radioactivity. WHO guidance advises against drinking water with radioactivity levels higher than 10 becquerels per litre.
Tepco says the radioactive water overflowed from a storage tank on Wednesday, but the leak was not discovered for several hours, the BBC's Rupert Wingfield-Hayes in Tokyo reports.
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