JAPANESE green tea, esteemed around the world for its purity and health-enhancing properties, has become contaminated with radiation, as fallout from the Fukushima Daiichi nuclear plant continues to blight Japan's agricultural heartlands, authorities revealed today.

Authorities admitted for the first time that green tea from Japan's biggest tea-growing area, the Shizuoka prefecture, contains radiation higher than the officially-permitted level.

The contamination has opened a furious argument among local and national officials about how to measure the radiation, and what constitutes a safe level of contamination.

Dried leaves from the year's first harvest in the Honyama area of Shizuoka were found to contain radioactive cesium at a level of 679 becquerels per kilogram, above the permitted maximum of 500 becquerels. But the discovery was made by chance, and the authorities admit that earlier consignments, which were not examined and have gone to the market, may have also been contaminated.

Limits on the sale of tea from areas closer to Fukushima have been put in place, but Shizuoka is to green tea what the Champagne region of France is to sparkling wine, and the effect of the news will be devastating.

Japan produced 86,000 tonnes of dried tea in 2009, and 42 per cent of that was from Shizuoka. The prefecture, supported by the ministry of agriculture, has insisted on carrying out radiation measurements in such a way as to minimize the suggestion that its precious product is dangerous.

The problem is that, unlike other vegetables, tea leaves are processed before going on sale and are not consumed directly. When fresh leaves are dried, the removal of water concentrates the radioactive elements to five times the former level.

But when they are infused in a tea pot the amount of radiation in the resulting brew is between 30 and 45 times less, according to the agriculture ministry.

The Shizuoka government wants the 500 becquerels limit to apply to the less intensely radioactive fresh leaves. But the health ministry argues that consumers might swallow dried leaves in a cup of tea, as well as in products derived from tea, such as green tea ice cream, and that the 500-becquerel limit for fresh vegetables must also apply to tea.

The high reading was discovered not by the tea grower or the local government, but by a mail order tea company in Tokyo that carried out its own measurements.

Originally published as Japan's green tea contaminated with radiation