A drug that can potentially cause cancer in humans may have entered the food chain via horses slaughtered in UK abattoirs, Labour claims.
Shadow environment secretary Mary Creagh said "several" UK-slaughtered horses had tested positive for the carcinogen phenylbutazone.
Agriculture minister David Heath said all meat was checked to ensure it was fit for human consumption.
The news comes after horse and pig DNA was recently found in some burgers.
Some of these were sold in Tesco, Iceland, Lidl and Aldi and Dunnes. Tesco took out adverts in British newspapers apologising for the matter.
There is no suggestion that these burgers contained phenylbutazone.
'Right to know'
Ms Creagh told Mr Heath in the Commons: "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."
When she asked if Mr Heath was aware of the cases, the minister replied: "The Food Standards Agency carry out checks in slaughterhouses to ensure that equine animals presented for slaughter are fit for human consumption in the same way as they do for cattle, sheep and other animals.
"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."
Ms Creagh then asked if that meant Mr Heath was aware of the issue.
"I'm astonished that you have not raised this and I think the public have a right to know," she said.
She also said the news was a "very serious development" and demanded action to ensure that "illegal and carcinogenic horsemeat stops entering the human food chain".
And she called on the government to reverse a "reckless" decision to end the National Equine Database.
But Mr Heath replied: "There is no difficulty in tracing the use of a horse passport. To suggest the National Equine Database was required to do that is simply erroneous."