Sixty-nine British sheep were rejected from the food chain in 2005 because they were too radioactive, according to new FSA figures.

Some 19 years after Chernobyl became a household name as the world’s worst nuclear accident, the Welsh animals emitted more than twice as much radioactivity as is safe for humans to consume.

They became contaminated by radiocaesium-137, emitted in a toxic cloud by the Chernobyl explosion in 1986 and deposited on upland grazing areas.

“The animals that failed did so by a considerable margin - we’re looking at thousands of Becquerels per kilogram above the 1,000Bq/kg considered safe for consumers,” said an FSA spokesman. “And while levels like this are being detected, it is vital testing continues to protect the industry, the Welsh lamb brand and consumers.”

More than 197,000 sheep on 371 farms in Britain will remain under restrictions. Numbers have barely fallen since 2000 and are mostly focused in north Wales, with some farms in south Cumbria and south west Scotland also affected.

Lambs and sheep that fail the test cannot be slaughtered for human consumption or for petfood and producers are compensated accordingly.

But some farmers are questioning the need to test so many animals given the low failure rate and cost entailed. Last year, 210,000 tests were carried out.

“I was shocked by the figures – particularly the number of Welsh flocks still being monitored,” said National Sheep Association chief executive Peter Morris. “This testing is inconvenient and it’s not good news for the producers involved.”

But Morris added the Welsh lamb brand had to be protected. “We fully support continued monitoring that is in proportion to any possible food safety risk.”