The operator of the crippled Fukushima nuclear plant in northern Japan says about 100 tonnes of highly radioactive water has leaked from a storage tank.
Tokyo Electric (Tepco) says it may have overflowed after a valve was left open by mistake, but although the leak was not discovered for several hours, the water is unlikely to have reached the ocean.
The plant, which was damaged by an earthquake and tsunami there years ago, has faced multiple problems including leaks and power cuts since the disaster and has never gone back into operation.
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.
Tepco says the water from Wednesday's leak gave a reading of 230 million becquerels per litre of radioactive isotopes. A becquerel is a unit used to measure radioactivity; the World Health organisation advises against drinking water with radioactivity levels higher than 10 becquerels per litre.
The company has apologised to the public and says it's now in the process of recovering the leaked water and the earth it has contaminated.
The 11 March 2011 tsunami knocked out cooling systems for the plant's reactors, leading to meltdowns at three of them. Water is being pumped in to cool the reactors but this creates large amounts of contaminated water that must be stored securely.