British supermarket chain Tesco said on Tuesday it had withdrawn a number of beef burgers from sale after samples were found to contain horse DNA in tests.
The burgers that had tested positive were produced in two plants in Ireland and one in the U.K. and sold by several retailers, the Food Safety Authority of Ireland said in a separate statement.
The Irish authorities said most of the affected burgers contained very low levels of horse DNA, but in one Tesco sample horsemeat accounted for about 29 percent of the content.
The case is potentially embarrassing for Tesco, which is fighting to turn around a slide in its market share by improving perceptions of its quality and service.
"We are working with the authorities in Ireland and the U.K., and with the supplier concerned, to urgently understand how this has happened and how to ensure it does not happen again," said Tesco in a statement on Tuesday evening.
Horsemeat poses no health risk to consumers, but culturally it is not eaten in the U.K. and Ireland.