Asda has been forced to recall thousands of packs of fresh lamb because they might have contained meat from sheep not fit for the food chain.

The sheep had been treated with a veterinary drug called Doramectin, which experts say causes sickness in animals and possibly humans. The recall covered 14 lines packed by Welsh Country Foods between 12-16 October, including halal and non-halal lamb legs, half legs, shoulders, necks and chops.

An Asda spokesman said the risk to the public was low and swift withdrawal was a precautionary measure. "Welsh Country Foods will be doing a root-and-branch review of how this happened," he said.

Welsh Country Foods said the 142 affected sheep only accounted for 0.6% of weekly throughput. "It is our intention to purchase lambs direct from farm where possible and reduce our reliance on livestock markets," said MD Trevor Hanger.