Levels of radioactive iodine in the sea near the tsunami-stricken Fukushima nuclear plant are 1,250 times higher than the safety limit, officials say.
The readings were taken about 300m (984ft) offshore. It is feared the radiation could be seeping into groundwater from one of the reactors.
But the radiation will no longer be a risk after eight days, officials say.
There are areas of radioactive water in four of the reactors at the plant, and two workers are in hospital.
The plant's operator says the core of one of the six reactors may have been damaged.
It has announced that fresh water rather than seawater will now be used to cool the damaged reactors, in the hope that this will be more effective.
Prime Minister Naoto Kan said the situation was "very unpredictable".
The official death toll from the 11 March earthquake and tsunami has passed 10,000, and more than 17,440 people are missing.
Hundreds of thousands of people have been made homeless; an estimated 250,000 people are living in emergency shelters. Food, water and fuel are in short supply.
The Japanese government has put the rebuilding cost at $309bn (£191.8bn).
I can't deal with what is happening lately there.
But at least now that there's an official report, people will completely stop eating the fish from that sea. Cause lately some news were reporting that some japanese were still waiting for confirmation.
God bless them.