Contaminated water with dangerously high levels of radiation is leaking from a storage tank at Japan's crippled Fukushima nuclear plant, the most serious setback to the cleanup of the worst nuclear accident since Chernobyl.

The storage tank breach of about 300 metric tons of water is separate from contaminated water leaks reported in recent weeks, plant operator Tokyo Electric Power Co said on Tuesday.

The latest leak is so contaminated that a person standing half a meter (1 ft 8 inches) away would, within an hour, receive a radiation dose five times the average annual global limit for nuclear workers.

After 10 hours, a worker in that proximity to the leak would develop radiation sickness with symptoms including nausea and a drop in white blood cells.

"That is a huge amount of radiation. The situation is getting worse," said Michiaki Furukawa, who is professor emeritus at Nagoya University and a nuclear chemist.

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