Japan is poised to impose a ban on shipments of cattle from Fukushima prefecture – the scene of its worst ever nuclear crisis – after discovering that meat containing abnormally high levels of radioactive caesium had been processed and consumed.
The cows had been fed on rice straw containing high levels of the radioactive isotope that was harvested after the 11 March tsunami triggered a core meltdown at Fukushima Daiichi nuclear power plant.
While officials said consuming the meat did not present an immediate threat to health, the incident has highlighted concerns over food safety in the wake of the crisis, which has caused contamination in milk, tea, leaf vegetables, fish and water.
The government is expected to announce the suspension of cattle shipments from Fukushima, and possibly other areas nearby, on Tuesday.
"The most likely outcome is that we will ban beef shipments," Goshi Hosono, a special adviser to the prime minister on the nuclear clean-up, said on a TV programme. "We are discussing the matter along these lines. We have to ensure food safety."
Authorities in the prefecture confirmed that 84 head of cattle from five affected farms had been shipped to eight locations, including Tokyo and Osaka, between late March and mid-July.
Local media reported that the contamination risk could be more widespread, affecting farms 60 miles from the power plant. Kyodo news agency said it had calculated that as many as 143 cows sent to all but 10 of Japan's 47 prefectures may have been exposed to radiation via feed.
"We may need to increase our response by checking the distribution of contaminated straw," said Kohei Otsuka, the senior vice-minister for health. "We are currently considering Fukushima prefecture, but we may have to consider the need for a further response by checking the distribution of contaminated straw."
Retailers said it was highly likely that some of the contaminated beef had already been eaten.
On Sunday, the Aeon chain of supermarkets said 319kg (703lb) of beef from a farm in Asakawa in Fukushima, had been sold at 14 of its stores in Tokyo and the surrounding area between late April and the middle of last month.
Tests on straw at a farm in Koriyama city in Fukushima prefecture showed caesium levels as high as 500,000 becquerels per kg. Those readings are about 378 times the legal limit set by the government.
Farmers in the area said they had not been told about a government warning, issued days after the nuclear accident, not to give their animals feed that had been stored outside.
The Tokyo metropolitan government said on Sunday that high levels of radioactive caesium were detected in meat from a cow shipped to a packing plant in Tokyo from a farm in Koriyama.
The meat contained radioactive cesium at measurements as high as 2,400 becquerels per kg, nearly five times the government-set safety limit of 500 becquerels per kg.
Efforts to track down exposed cattle began earlier this month after meat from cows at a farm in Minamisoma, about 15 miles from Fukushima Daiichi, were found to have been fed with contaminated straw.
Authorities in Tokyo said they had detected radiation levels in beef originating from the farm of up to 2,300 becquerels per kg.
Government officials and some experts played down the risk to health. "This is not a number that would clearly cause abnormal effects on health even if the beef was eaten," Ikuro Anzai, honorary professor of radiation protection at Ritsumeikan University, told Kyodo.
But he added: "It would be better to refrain from eating it until the situation becomes clear."