The shadow environment secretary has warned carcinogen-contaminated horse meat could have entered the food chain.
Speaking to the Commons on Thursday, Mary Creagh said she had evidence "several" horses slaughtered in UK abattoirs had tested positive for phenylbutazone.
There is nothing to suggest the issue is linked to the scandal earlier this month where horse meat was found in the burgers of a number of retailers including Tesco and Lidl.
The drug, bute for short, is an anti-inflammatory used to treat pain and fever in animals but can cause cancer in humans and suppress white blood cell production.
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."
Agriculture minister, David Heath, insisted the Food Standards Agency (FSA) checked all meat slaughtered in the UK to ensure it is safe to eat.
He added: "In addition, the FSA carry out subsequent testing for phenylbutazone and other veterinary medicines in meat from horses slaughtered in this country.
"Where positive results for phenylbutazone are found, the FSA investigates and takes follow-up action to trace the meat."
Creagh said she was "astonished" at the "very serious development" and demanded further action to stop carcinogens entering the food chain.
Burger King announced on Thursday it was dropping one of the suppliers involved in the horse meat burger scandal.