Japan is considering a ban on the shipments of all cattle from Fukushima prefecture, following the news that contaminated beef from 42 cattle has been shipped to Tokyo and other areas over recent months.
The animals were fed straw containing radioactive caesium more than 70 times the legal limit.
The revelation pointed at gaps in food supply monitoring since the March 11 quake and tsunami disaster sparked the world's worst nuclear accident since Chernobyl 25 years ago, contaminating the air, soil and sea.
Fukushima prefecture, which hosts the stricken atomic power plant, reported that a farm in Asakawa, about 60 kilometres (40 miles) from the plant, had shipped the beef to Tokyo and elsewhere between April 8 and July 6.
The straw the cattle were fed had been left in an open field and contained up to 97,000 becquerels of caesium per kilogram -- more than 70 times the government-designated limit, the prefecture said in a statement.
The Tokyo Metropolitan Government said that one beef sample tested at 650 becquerels per kg, exceeding the maximum limit of 500 becquerels per kg.
It was the third shipment of radioactive beef reported since last weekend, and officials believe much of it has already been consumed.
A cattle farm in Minamisoma, just outside the 20 kilometre no-go zone around the nuclear plant, was found to have shipped meat from cows fed with straw containing 75,000 becquerels of caesium per kg.
The government has sought to assure the public that there is no immediate health threat from eating standard servings of the beef.
More than four months into the nuclear crisis, Japan has not set up a centralised system to check food for radiation, relying instead on testing carried out by prefectures and municipalities.