In a test by the Tokyo Electric Power Company, "radioactive iodine-131 at 1,250.8 times the legal limit was detected several hundred metres offshore near reactor number one," an agency official told AFP.

Another agency spokesman, Hidehiko Nishiyama, in a televised press conference said that the level is "relatively high" but said that the impact on marine life and seafood would be minor. "This figure means that if you drank 500 millilitres (17 fluid ounces) of water containing this level of iodine it would reach the limit that a person can take in in one year, which is one millisievert.

"This is a relatively high level." Speaking on the likely impact on aquatic life, Nishiyama added: "Generally speaking, radioactive material released into the sea will spread due to tides, so you need much more for seaweed and sea life to absorb it.

"And, since (the iodine) has a half-life of eight days, by the time people eat the sea products its amount is likely to have diminished significantly." The new reading, taken yesterday, is sharply higher than several taken last week at the same spot, about 330 metres offshore.

TEPCO said Tuesday that the seawater reading was 126 times above the legal level, and on Thursday that it was 145 times the legal level. Its latest tests also detected radioactive caesium-134 that was 117.3 times the legal limit at the same location. Levels of caesium-137, which has a half life of about 30 years, was 79.6 times the legal maximum, it said.

Fire-engines and concrete trucks have poured thousands of tons of seawater onto the reactors and into fuel rod pools at the plant after cooling systems were knocked out by the March 11 quake and tsunami.