It is the first time radiation levels this high have been found in rice since the nuclear disaster in March.

The tests were carried out on rice harvested from a field 50 kilometres from the nuclear plant.

The levels of radioactive caesium were measured at 630 becquerels per kilogram, above the maximum allowable level of 500 becquerels.

Officials from Fukushima prefecture have now asked all rice farmers in the district to suspend shipments.

The Fukushima agriculture department says it is appalled by the results and it will retest 154 farms in the area.

The findings will further worry nervous consumers, already fretting over the safety of domestic produce despite its previous solid reputation.

There have been a series of scares over radiation in food in Japan in recent months; in products such as beef, mushrooms and green tea, but never before in the country's staple, rice.

Japanese-grown grain is widely held to be superior to imports and is heavily protected by massive tariffs aimed partially at propping up the nation's ageing farmers.

The rice was being prepared for market, but the chief cabinet secretary, Osamu Fujimura, says none had been sold.

The discovery highlights the difficulty of tracking the radiation, which has been spread across eastern Japan by wind and rain.

If a ban is imposed it would be the first on rice shipments since the Fukushima plant was crippled by a massive quake and tsunami on March 11.

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