Britain's Food Standards Agency (FSA) said the level of the drug, which is banned from entering the human food chain, posed a low risk to human health, as results showed it contained 4 parts per billion.
Asda, the British arm of the U.S. retailing giant Wal-Mart , said the drug had been found in tins of its Smart Price Corned Beef which had tested positive for horsemeat DNA in March and was removed from its selves.
"Although there is a very low health risk, we are recalling this product," it said. The FSA said people who still had the affected corned beef should not eat it but return the tins for a refund.
Bute, a commonly used medicine in horses, can be harmful to humans but only in very high concentrations.
"Animals treated with bute should not enter the food chain as the drug may pose a risk to human health," the FSA said.
"However, even if people have eaten products which contain contaminated horsemeat, the risk of damage to health is very low."
Europe's horsemeat scandal erupted in January, when testing in Ireland revealed that some beef products also contained equine DNA.
In February, the French farm minister said meat from three horse carcasses contaminated with bute had entered the human food chain in France but added there was no public health risk.
The affected carcasses were from horses slaughtered in Britain which had been exported to France.
