More than six times the legal limit of radioactive caesium has been found in beef from Fukushima prefecture, home to Japan's crippled nuclear plant, an official statement said Saturday.

The meat came from one of 11 cows shipped this month to Tokyo from a farmer in Minamisoma city, according to the statement by the Tokyo metropolitan government.

The 11 cows all showed high levels of radioactive caesium, ranging from 1,530 to 3,200 becquerel per kilogram, compared with the legal limit of 500 becquerel, the Tokyo statement said.

It was the first time excessive levels of radioactive caesium have been found in meat, according to a Tokyo official.

"All the meat from the cows is kept in the laboratory and has not entered the market," a separate statement said.

But a Tokyo official said five cows from the same farmer in Minamisoma have already been sold into the Tokyo market on May 30 and June 30, with the metropolitan government trying to track them down.

Following the latest inspection results, the government of Fukushima prefecture immediately requested that the city of Minamisoma refrain from shipping cows and beef, a Fukushima official said.

Fukushima prefecture has checked radiation levels of the outer skin of all livestock shipped from areas near the troubled nuclear plant, and the 11 cows had cleared external inspections, Jiji Press said.

The city of Minamisoma lies on the outskirts of the Fukushima Daiichi nuclear power plant, which has leaked radioactive substances into the environment after the March 11 tsunami and earthquake crippled its cooling systems.

The 11 cows were raised and shipped by the farmer just outside of the 20-kilometre (12-mile) no-go zone around the plant, the statement said.