Tesco's Group Technical Director Tim Smith said the supermarket was "appalled" over the burgers that contained 29% horsemeat and they took "full responsibility" for the error.
Mr Smith told the Commons Environment Committee that the supermarket would "continue to investigate as though Tesco and not the supplier was responsible."
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The Office for National Statistics said gross domestic product grew by just 0.1%.
William's comments came during a surprise appearance at an event scheduled for just his brother.
Matt Campbell, who collapsed just a few miles from the finish line, had hoped to raise £2,500.