Burgers contaminated with horsemeat could contain a drug linked to cancer, consumers were warned today.
Five horses slaughtered in the UK last year tested positive for phenylbutazone, food safety chiefs admitted.
The anti-inflammatory is banned from the food chain over fears it causes bone marrow and liver problems in humans.
But test delays meant meat from the five contaminated horses still ended up on dinner plates.
The new scare comes after four British supermarkets, including Tesco, were found to have sold beefburgers containing horse.
The Food Standards Agency tonight stressed that no burgers had tested positive for phenylbutazone – also known as Bute.
But Shadow Environment Secretary Mary Creagh warned that contaminated horse could have ended up in other batches.
In the Commons she demanded action to ensure “illegal and carcinogenic horsemeat stops entering the human food chain”.
And she told Agriculture Minister David Heath: “I’m astonished you have not raised this. The public has a right to know.”
Last week supermarkets were forced to withdraw more than 10 million burgers containing Irish horsemeat.
At the time experts said it posed no health risk.
All Irish and UK horses must have a “passport” which is supposed to stop those that have had dangerous drugs from being eaten.
But 75 different UK organisations issue them and a national database has been closed by the government.
An animal treated with a drug such as Bute is worthless. But with a fake passport it can fetch more than £300 from the meat trade.
The UK Food Standards Agency said that of 8,426 horses slaughtered here last year only 145 were tested.
Of these five were contaminated with Bute but it was too late to stop them being eaten in France.
The spokesman added: “Tests take three weeks. Because suppliers want the meat fresh it did go into the food chain. We alerted the French.”
Unison, which represents meat hygiene workers, called for inspections to be stepped up.
Spokesman Ian Adderley said: “Just a small number of horses are checked and the results take weeks to come back.
“They are playing Russian roulette with consumers.”
The Department for Environment, Food and Rural Affairs claimed that the horse database would not have helped because it did not include veterinary records.
They added: “Anyone falsifying horse passports can be prosecuted.”
Leeds University’s Prof Alastair Hay said Bute could cause anaemia and bone marrow disorders but the cancer link had not been convincingly proven.
What is Phenylbutazone?
Phenylbutazone, aka Bute, is an anti-inflammatory drug given to horses to treat lameness, pain and fever.
It is banned from entering the human food chain in Europe because it is thought to cause bone marrow disorders in extremely rare cases.
But scientists are still divided about how dangerous the drug could be to humans.
Professor Chris Elliott, director of the Institute for Global Food Security at Queen’s University Belfast, explained: “Thankfully the residues of drugs such as this that are found in meat are very low, and therefore the risk to the consumer is correspondingly low.
“However, the use of veterinary medicines in all animal species that go into the food chain is a matter of food safety and must be treated seriously.”