LONDON (AP) - Six horse carcasses that tested positive for an equine painkiller may have entered the human food chain in France, Britain's food regulator announced Thursday - and the agency's chief said horsemeat tainted with the medicine may have been sold to consumers "for some time." The Food Standards Agency said eight out of 206 horses it checked had tested positive for phenylbutazone, commonly known as bute. It said of those eight, six - all slaughtered by a firm in southwest England - were sent to France and "may have entered the food chain." The agency said it was working with French authorities to trace the meat. Environment Minister David Heath earlier told the House of Commons three horses might have entered the food chain, but the ministry later said six was the correct figure. Britain's chief medical officer, Sally Davies, insisted that horsemeat containing bute "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-percent 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. Authorities across Europe are testing thousands of meat products for the drug, and for horse DNA, after horsemeat was found in food products labeled as beef in several countries. Food Standards Agency head Catherine Brown said that before the current crisis, the agency had tested about 5 percent of the horses slaughtered in Britain - and about 6 percent of those had shown traces of bute. "That would say there has been a significant amount of carcasses with bute in going into the food chain for some time," she said. Pan-European police agency Europol is coordinating a continent-wide fraud investigation amid allegations of an international criminal conspiracy to substitute horse for more expensive beef. The scandal has uncovered the labyrinthine workings of the global food industry, where meat from a Romanian slaughterhouse can end up in British lasagna by way of companies in Luxembourg and France. It has also raised the uncomfortable idea that Europeans may unwittingly have been consuming racehorses, which are often treated with bute. Britain's Food Standards Agency said it had begun testing all horses slaughtered in Britain for bute, and that none would be exported for consumption unless they tested negative. The agency previously tested only a small percentage of slaughtered animals, which has fueled criticism of its failure to catch the horsemeat contamination sooner. Almost no horsemeat is consumed in Britain, where hippophagy - eating horses - is widely considered taboo. But thousands of horses killed in the country each year are exported for meat to countries including France and Belgium, which have a culture of eating horsemeat. A "horse passport" system, which records whether animals have been treated with bute, is meant to stop the drug entering the human food chain. On Thursday, Britain's Aintree race track said a slaughterhouse in northern England shut down this week by government investigators had a contract to dispose of fatally injured racehorses. The racecourse said it was "as confident as we possibly can be" that none of the meat had entered the human food chain. The trail of illicit horsemeat stretching across Europe spread still further Thursday when Rangeland Foods, a processing factory in Ireland, said it had withdrawn some batches of burger products which contained beef supplied from Poland after it tested positive for up to 30 percent horse meat. Food Safety Authority of Ireland said the products had been sold to the catering and wholesale sectors and distributed to Ireland, Britain, Spain, France, Germany and the Netherlands.