One of Britain's leading supermarkets, Tesco, announced that traces of horse DNA are in the hamburgers on sale in the U.K. and Ireland, NBC News reported. The discovery was made by the Food Safety Authority of Ireland, which said it carried out a study to examine the "authenticity" of several beef burger, beef meal, and salami products. Ten of the 27 beef burgers tested contained horse DNA, with nine containing only "very low levels." Twenty-three of the 27 burgers also tested for positive for pig DNA and and 21 out of 31 “beef meal products” tested were also found to contain pig DNA, but no horse DNA was discovered. Tim Smith, Tesco’s group technical director, said the store apologized “sincerely for any distress” caused. “We immediately withdrew from sale all products from the supplier in question,” he said. "The presence of illegal meat in our products is extremely serious. Our customers have the right to expect that food they buy is produced to the highest standards." An FSAI official said there is no clear explanation for the presence of the horse DNA.