Environment Minister David Heath told the House of Commons that eight horses from British abattoirs had tested positive for bute, and "three may have entered the food chain in France. The remaining five have not gone into the food chain."
Horsemeat itself is not dangerous to eat, though it is widely considered taboo in Britain and Ireland. But bute, or phenylbutazone, a painkiller and anti-inflammatory used on horses, is considered harmful to human health if ingested.
Heath said frozen lasagna sold under the Findus label in Britain had tested negative for bute. The meals were removed from store shelves last week after tests found some contained more than 60 per cent horsemeat.
Authorities across Europe are testing thousands of meat products for the drug after horsemeat was found in food products labeled as beef in several countries.
Britain's chief medical officer, Sally Davies, insisted that horsemeat containing phenylbutazone "presents a very low risk to human health."
Davies said the drug is occasionally prescribed to patients suffering from severe arthritis, and while it sometimes produces serious side effects, "it is extremely unlikely that anyone who has eaten horsemeat containing bute will experience one of these side effects."
"If you ate 100-per cent horse burgers of 250 grams (8.8 ounces), you would have to eat, in one day, more than 500 or 600 to get to a human dose," she said. "It would really be difficult to get up to a human dose."