Justin McCurry in Tokyo
The firm said that as much as 45 tonnes of water had leaked through a crack in the concrete wall of a building being used to purify contaminated water that is then used to cool molten fuel in the plant's three damaged reactors.
The firm has piled up sandbags to prevent further leaks but fears that some water may have already found its way into a gutter that connects to the Pacific ocean about 600 metres away.
Experts believe the water could contain high levels of strontium, a beta-emitting radioactive substance that, if ingested, can cause bone cancer.
Public broadcaster NHK reported that although caesium levels in the leaked water were low, it could contain up to 130,000 becquerels per cubic centimetre of strontium, which has a half-life of 29 years