LONDON – In Britain, a horse is a horse – not a main course.

Tesco, the country’s biggest supermarket chain, took out full-page newspaper ads Thursday to apologize for an unwanted ingredient in some of its hamburgers: horsemeat.

Ten million burgers have been taken off shop shelves after the revelation that beef products from three companies in Ireland and Britain contained horse DNA. Most had only small traces, but one burger of a brand sold by Tesco had meat content that was 29 percent horse. The contrite grocer told customers that “we and our supplier have let you down, and we apologize.”

Reaction to the scandal in Britain goes beyond concerns about contaminated food. While people in some countries happily dine on equine flesh, in the land of Black Beauty and “National Velvet,” the idea fills many with horror.

Mary Creagh, environment spokeswoman for the opposition Labour Party, reflected the feelings of many when she said Thursday that eating horsemeat is “strongly culturally taboo in the United Kingdom.”

She was echoing prohibitions in Western cultures that date to A.D. 732, when Pope Gregory III declared horse-eating a pagan practice.

Horsemeat has never been a staple of European diets, but from the mid-19th century, it was eaten in countries including Britain as cheap filler food for the poor.

“It tended to be in burgers and potted meats and sausages as cheap supplementary food,” culinary historian Annie Gray said. “And it wasn’t always labeled, just as we’re finding out at the moment.”

The sale of horsemeat in the U.K. continued through the 1930s Depression and World War II, when many foods were rationed. But hippophagy – eating horses – never really caught on in Britain, a land of horse lovers and beef eaters.

“The eating of beef is totally symbolic of being English,” Gray said – so horse was always going to be seen as a poor substitute.

Horsemeat is eaten in European countries such as France, Belgium and Italy, as well as by many in China, among the traditionally nomadic people of Central Asia and in parts of Latin America.

In France, specialist horse butchers and supermarkets sell “viande chevaline.” There and in Belgium, it’s popular both cooked and raw, in a form of steak tartare.

Some of the horsemeat eaten in mainland Europe comes from Britain, whose love of horses doesn’t stop it from sending thousands of horses a year abroad to be killed for meat.

Fans of horsemeat say it is extremely healthy: low in cholesterol and fat, high in protein and omega-3 acids.

“I think it’s delicious,” said Caroline Roddis, a freelance writer who organizes Flogging a Dead Horse, a series of dining events at which horsemeat is served. “It is slightly sweeter than beef and it has got such a good depth of flavor, it is hard not to like it.”

Roddis reports a healthy interest in her London events, which draw as many as 50 British and foreign diners a night to sample the delights of horseflesh.

She hopes the “horseburger” scandal will make people think more about what they eat.

Officials say the source of the contamination may be a powdered beef-protein additive imported from Spain and the Netherlands to pad out the cheapest burgers, which typically contain between 60 percent and 70 percent meat alongside flour, water and other fillers.

“We don’t really think (about) what we are putting in our mouths,” Roddis said.

The handful of hardy entrepreneurs who sell horsemeat in Britain say the scandal has – paradoxically – been good for business, raising the profile of a meat few had considered.

“We’ve been very busy the last couple of days,” said Paul Webb, director of Exotic Meats, a company that sells horse burgers, sausages and steaks alongside cuts of crocodile, kangaroo and impala.