Grocery retailer Asda has been forced to withdraw packs of lamb from store shelves across the country after the meat from 142 animals was found to contain a drug for treating parasites.

The affected meat, which includes nearly all types of cut, was supplied by Welsh Country Foods in packs displaying dates between 12-16 October.

The sheep had been treated with Doramectin prior to being sold before the meat withdrawal period had passed. According to the Food Standards Agency, the lamb may have come from animals sold for breeding purposes, not from those intended for the food chain, but this remains to be confirmed.

The FSA praised Asda’s response, which included removing potentially affected meat and erecting in-store notices that advised consumers not to consume it and return it to the store for a refund. Chris Brown, Asda’s head of ethical and sustainable sourcing, said “a very small amount” had been affected, but conceded the exact quantity was unknown as investigations were ongoing.

The 142 potentially affected lambs were purchased from livestock markets and represent less than 0.6% of WCF’s weekly processing throughput.

Trevor Hanger, managing director of Welsh Country Foods, said: “We have been fully co-operating with the Food Standards Agency over this matter. We are confident that our traceability records have identified all the likely destinations for this lamb.

“Following guidance from the FSA we have contacted all of our potentially affected customers about the situation and have instructed them to withdraw these products from sale.”

He added: “We are conducting a full and thorough review of our lamb procurement process. As part of our procurement policy it is our stated intention to purchase lambs direct from farm when and where possible, reducing our reliance on livestock markets.”

The health concerns surrounding Doramectin relate to repeated exposure that has caused sickness, and other effects, in laboratory animals.