Shadow Environment Secretary Mary Creagh said:
"I am in receipt of evidence showing that several horses slaughtered in UK abattoirs last year tested positive for phenylbutazone, or bute, a drug which causes cancer in humans and is banned from the human food chain. It is possible that those animals entered the human food chain."
A drug with the potential to cause cancer in humans might have entered the food chain through horse meat, Labour has claimed.
Traces of horse DNA have been discovered in a beef burgers sold in supermarkets across the UK. A Tesco burger was found to be 29% horsemeat
A list of the beef burgers, sold in UK supermarkets, that were found to contain traces of horsemeat.