Asda is recalling all corned beef from its budget range after traces of veterinary drug phenylbutazone were found in some batches.
The Food Standards Agency has confirmed that "very low levels" of the painkilling medicine, known as bute, were detected in the Asda Smart Price Corned Beef.
Customers who have bought the 340g tins, with any date code, have been urged not to eat the corned beef but to return it to the supermarket.
Asda withdrew the product on March 8 after it was found to contain more than 1% horse DNA. Bute was detected in some samples, at the level of four parts per billion (4ppb), when further tests were carried out.
The corned beef is the only meat product in which bute has been found, according to the FSA.
Bute had been discovered in horse carcasses in February however, with the highest level found being 1900ppb.
The FSA said no other Asda products are thought to be affected and that customers who bought the corned beef should contact the supermarket for a refund.
They said that while animals treated with bute should not enter the food chain, the risk of damage to the health of anyone who had eaten such meat is "very low".
Chief Medical Officer Professor Dame Sally Davies previously said: "Horse meat containing phenylbutazone presents a very low risk to human health.
"Phenylbutazone, known as bute, is a commonly used medicine in horses. It is also prescribed to some patients who are suffering from a severe form of arthritis."
She said the levels of bute previously found in horse carcasses meant a person would have to eat up to 600 burgers, containing 100% horse meat, every day to come close to consuming a human's daily dose of the drug.
Dame Sally added: "In patients who have been taking phenylbutazone as a medicine there can be serious side effects but these are rare. It is extremely unlikely that anyone who has eaten horse meat containing bute will experience one of these side effects."
Horse carcasses in the UK need to have a negative bute test before they can enter the food chain.
In a statement on the Asda website, the supermarket said it was also recalling tins of Chosen By You corned beef.
"The tinned Chosen By You Corned Beef (340g) product, also withdrawn in March, has not tested positive for phenylbutazone," the statement read.
"However as a precaution it is also being recalled as it is made in the same factory."
Asda claimed to have taken "an extremely cautious approach since the very beginning" and had carried out more than 700 tests so far, "moving swiftly to remove any products" when they had any concerns.
The supermarket added: "The FSA has reassured us that the quantities we've found pose a low risk to human health."
Shadow environment secretary Mary Creagh said: "It is deeply worrying that bute, a drug banned from the human food chain, has been discovered in one brand of corned beef.
"This product was withdrawn from sale on March 8 yet has only been formally recalled now, after testing positive for bute, meaning people could have unwittingly been eating meat containing this drug for the last month.
"This exposes the weaknesses in the Government's handling of the horsemeat scandal where products were withdrawn but in some cases not tested either for horsemeat or bute. The interests of the consumer should have been put first."
In a separate development earlier in the day, the FSA said that two more beef products had been found to contain horse DNA as part of the UK-wide sampling they had undertaken.
The FSA confirmed results for four of the remaining five samples tested under the programme, adding that all five products had already been removed from sale.
Neither of the two samples containing horse DNA at or above the 1% threshold for reporting - a burger bought from Nefyn Pizza and Kebab House in Gwynedd and manufactured by the Burger Manufacturing Company and a beefburger bought from Pig Out in Walsall and manufactured by King Fry Meat Products - were found to contain bute or pig DNA.
There is one result yet to be reported.
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