Horse meat infected with a hazardous drug entered the food chain after being slaughtered in Britain.
Eight equine carcasses tested by the Food Standards Agency were to find to contain traces of phenylbutazone - or bute.
Three of them may have entered the human food chain in France, where horsemeat is a regular feature on plates around the country.
Three other horse carcasses were shipped across the channel, but are not at risk of being eaten by people.
The remaining two carcasses are still in the UK, Food and farming minister David Heath MP told parliament.
England's chief medical officer said the risk to humans from the meat infected with bute was "very low."
Dame Sally Davies said: "If humans have eaten contaminated meat there is a very low risk. In the recent results the amount of bute is between trace and 1.9mg per kilogram.
"A person would have to eat between 500 and 600 100percent horsemeat burgers in one day to get a treatment does of Phenylbutazone."
Bute can cause anaemia in some people and halt white blood cell production in the body.
Heath said "the biggest investigation ever" was underway into adulterated meat on sale in products on supermarket shelves.
Tests were carried out on 206 horse carcasses between January 31 and February 7, said Heath. That was before packets of Findus lasagne were found to contain 100 percent horsemeat.
The coalition government has been criticised for "catastrophic complacency" by Labour critics.
Shadow environment secretary Mary Creagh tabled an urgent question in parliament on the horsemeat crisis.
"I raised the problem of bute-contaminated horsemeat being released into the food chain with you at Defra questions last month," she said.
"What action did you take with the FSA to reassure yourself after I raised those concerns? Were you aware of bute contamination before that day?
"Can you explain why until four days ago all horses were being tested for bute in this country but still released for human consumption?"
Heath said Creagh had failed to share her findings on bute. That triggered a furious reaction from Creagh, who was prevented from responding by the Speaker.