Some 300 tonnes of radioactive water is believed to have leaked from a tank at Japan's crippled nuclear plant, the worst such leak since the crisis began, the operator said today.

Tokyo Electric Power Company (TEPCO) said the leak was believed to be continuing Tuesday at Fukushima and it had not yet pinpointed the source of it.

The company later said it had identified which tank was faulty but had yet to find the spot from where it was leaking.