Japan banned shipments of rice grown near a tsunami-hit nuclear power plant for the first time on Thursday after detecting radiation exceeding the legal limit.
Chief Cabinet Secretary Osamu Fujimura said a sample of rice from a farm contained 630 becquerels of cesium per kilogram.
Cesium is among the radioactive materials that leaked from the Fukushima Dai-ichi nuclear plant after it was damaged by the March 11 earthquake and tsunami which left 20,000 people dead or missing.
Under Japanese regulations, rice with more than 500 becquerels of cesium per kilogram is not allowed to be consumed.
Officials have tested rice at hundreds of spots in Fukushima, and none had previously exceeded the limit.
Fukushima only last month declared that rice grown in the prefecture was safe.
A there have been a series of scares over radiation in food in Japan in recent months — in beef, mushrooms and green tea among other products — but never before in the country's staple, rice.
The report said the discovery highlights the difficulty of tracking the radiation, which has been spread across eastern Japan by wind and rain.