Horse DNA was found in 10 out of 27 hamburgers sold in supermarkets in Ireland and Britain after testing conducted by the Irish Food Safety Authority, 23 of them contained pig DNA.
The tainted meat came from two processing plants in Ireland and one in Britain but cause no public health risk, according to the Irish food authority.
"This raises concerns in relation to the traceability of meat ingredients and products entering the food chain," the authority said. They also reported that horse DNA was found at very low levels in nine out of 10 of the burger samples.
But burgers from Britain's biggest vendor, Tesco, pulled their product from supermarket shelves after the Irish Food Authority found 29 percent of horse meat in just one sample.
Tesco attempted to quell the situation with a statement.
"We understand that many of our customers will be concerned by this news, and we apologize sincerely for any distress," Tesco said.
The company also said that they were working with their supplier and Irish and British authorities to sort out the issues.
Chief executive of the Irish Food Safety Authority Alan Reilly said that it was not out of the ordinary to find pig DNA in their testing because different animals are processed in the same plant. But he could not offer an explanation for the presence of horse meat in the burgers.
"In Ireland, it is not in our culture to eat horse meat and therefore, we do not expect to find it in a burger," Reilly said. "Likewise, for some religious groups or people who abstain from eating pig meat, the presence of pig DNA is unacceptable."