TOKYO - Japan's Meiji Holdings said on Tuesday that radioactive cesium was found in infant milk powder made by the food and dairy firm, in the latest food scare to grip the country nearly nine months after the Fukushima nuclear disaster.

Shares of Meiji plunged nearly 10 percent to their lowest close since May 2009 following the news. Meiji said it was recalling 400,000 cans of the infant formula, which is sold only in Japan.

Worries over the safety of food supplies have shaken the public after the March 11 earthquake and tsunami crippled the Fukushima Daiichi plant in the worst nuclear accident in 25 years, spreading radiation over a large swathe of northern and eastern Japan.

Cases of excessive radiation in vegetables, tea, milk, seafood and water have stoked anxiety despite assurance from public officials that the levels detected are not dangerous.

Meiji said it is unsure exactly how the cesium got into the powdered milk, but it suspects radioactive substances emitted from the Fukushima accident may have been the source. A company spokesman told Reuters hot air used in the drying process may have contained cesium.

Tests by Meiji showed that up to 30.8 becquerels of cesium was found per kilogramme of the powdered milk. That is below the government-set permissible limit, but the firm will nevertheless conduct a voluntary recall of the product, called "Meiji Step."

The limit set by Japan's health ministry is 200 becquerels per kilogramme for powdered milk, an official at the ministry's department of food safety said.

Tokyo Electric Power (Tepco), the operator of the Fukushima plant, has made slow but steady progress in bringing the facility under control, curbing the amount of radiation emitted from its reactors and reducing temperatures of the water cooling them to levels considered stable.

It is on track to declare a "cold shutdown" -- when water used to cool the reactors is stable below boiling point -- before the end of the year.

Tepco said this week about 45 tonnes of contaminated water had leaked from a system that cleans radiated water, of which the utility said 300 litres escaped outside. But trade and energy minister Yukio Edano told reporters on Tuesday that the leak would not affect the goal of achieving a cold shutdown before the year's end.