REPORTER: British supermarket chain Tesco has cleared its shelves of its own brand "beef" burgers. They're not a health hazard but they were found to contain horse meat or traces of it - something that's not socially acceptable in the UK. They were one of five major supermarket chains affected, including Aldi. Three meat factories are now at the centre of an investigation - one in Ireland, two in England. In most cases, levels were low but one Tesco sample contained almost a third horse meat. Tim Smith is the group's technical director.
TIM SMITH, TESCO TECHNICAL DIRECTOR: "There are really only two ways that this could happen, one of them involves illegality by suppliers or suppliers to those individual suppliers or gross negligence, in which case we're still very keen to get to the bottom of this."
REPORTER: The world's third largest retailer is already fighting to regain customer confidence and market share after a tough 2012. The news knocked 1.7 percent off its shares.
WOMAN: "If the likes of Tesco's are doing it, who can you trust?"
MAN: "If it's meant to be a chicken burger or a beef burger and it's got traces of horse meat in, that's not on."
REPORTER: It's not yet known how the meat got into the supply chain. It could have been contaminated in transit or deliberately smuggled into the factories. Food campaigner Jeanette Longfield says the supermarkets should have had stricter controls.
JEANETTE LONGFIELD, ALLIANCE FOR BETTER FOOD AND FARMING: "I would have expected them to be doing much more routine testing and to have found this before, so that is another rather worrying development."
REPORTER: Tesco is investigating and while horse meat poses no risk to humans it does raise the question - what else are we eating we don't know about?