AROUND 100 tonnes of highly contaminated water have leaked from a storage tank at Japan's Fukushima nuclear plant, its operator has said.

The incident is the worst since last August, when a series of similar leaks sparked international alarm.

Tokyo Electric Power Co (Tepco) said the latest leak was unlikely to have reached the ocean.

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However, news of the leak at the site, which was devastated by the 2011 earthquake and tsunami, is likely to further erode public trust in a utility which has been rocked by a string of mishaps and official criticism of its clean-up operation.

Masayuki Ono, a spokesman for Tepco, said: "We are taking various measures, but we apologise for worrying the public with such a leak."

The firm said water overflowed from a storage tank at the site late on Wednesday after a valve had been left open by mistake and sent too much contaminated water into a separate holding area.

A worker patrolling the area, around 700 yards from the ocean, spotted water leaking through a drain attached to the side of the tank.