Tim Smith, group technical director of Tesco has apologised to customers after beef burgers sold at the supermarket were found to contain horse meat.
He said that there is no way to know until investigations have been completed how this could have happened. He added "there are only two ways that this could happen, one of them involves illegality by suppliers or the suppliers of those suppliers or gross negligence."
The horsemeat scandal demonstrates how long and complicated the food chain we all depend on is. Regulating our food supply is expensive.
Some of the best, and worst, jokes and gags from Twitter users discussing the discovery of horse DNA in some supermarket beef burgers.
We've seen the jokes on Twitter but the discovery of horse and pig DNA in beefburgers poses a serious question; where did it come from?