A banned drug has probably entered the food chain through horses, officials have admitted.
The Food Standards Agency said the painkiller bute was found in eight horses slaughtered in the UK. Two were retrieved, but the other six have not been traced.
The FSA said bute which is potentially dangerous for humans, could have been entering the food chain in a "significant amount" of carcasses.
However, it has also been revealed that bute was not found in tests on Findus lasagne, which was withdrawn from sale because it contained horse meat.
And the country's chief medical officer said a person would have to eat 500 horse burgers every day to consume a harmful dose.
Speaking at a press conference, FSA chief executive Catherine Brown said authorities in Britain and France are trying to track down the carcasses which may have entered the human food chain across the Channel.
A total of 206 horses tested by the Food Standards Agency in the first week of this month. Two were intercepted and destroyed before leaving the slaughterhouse but the other six were sent to France, where horse meat is commonly eaten.
Brown said: "That would say there has been a significant amount of carcasses with bute in going into the food chain for some time."
It is the first time that officials have admitted phenylbutazone - or 'bute' - could have entered the food chain.
The six bute-contaminated horses which were sent to France had been slaughtered by LJ Potter Partners at Stillman's (Somerset) Ltd in Taunton, Somerset, said the FSA. The remaining two, slaughtered at High Peak Meat Exports Ltd in Nantwich, Cheshire, did not leave the slaughterhouse and have been destroyed.
Chief Medical Officer Dame Sally Davies said that although the drug was linked to side-effects in patients who have been taking it as a medicine for arthritis, the risk was very low.
She said: "If you ate 100% horse burgers of 250g, you would have to eat, in one day, more than 500 or 600 to get to a human dose.
"It would really be difficult to get up to a human dose."
Announcing the results of the bute tests in the House of Commons, agriculture minister David Heath said the Government had instigated the "biggest investigation ever" into criminal activity in Europe over horsemeat contamination of beef products.
The FSA is set to report on UK products on Friday after retailers and suppliers were asked to provide "meaningful results" from tests to detect the presence of horsemeat in processed meals labelled as beef.
But shadow environment secretary Mary Creagh accused the Government of "catastrophic complacency" over the danger of bute entering the human food chain.
Reminding Heath that she had raised the issue with him in the Commons last month, Ms Creagh said she was "astonished" to learn that contaminated horsemeat may have been sent to France for human consumption.
"We must make sure horsemeat intended for humans is not contaminated with bute, it really is as simple as that," she said.
"Why did you not act immediately when I raised this issue three weeks ago in this House?"
Prime Minister David Cameron's official spokesman said: "Bute should not be present in horses that go into the food chain. It is incredibly important that we get to the bottom of what is happening.
"My understanding is that we are working very closely with French authorities on tracing the three carcasses involved that went to France. We are doing that as a matter of urgency with the French authorities."
EU ministers agreed at an emergency summit in Brussels last night to the random testing of meat products across Europe for both horse DNA and bute.
But a parliamentary report today condemned the Government for its "flat-footed" handling of the scandal, warning that its ability to respond has been weakened by cuts at the FSA.
The Commons Environment, Food and Rural Affairs Committee said the public appeared to have been "cynically and systematically duped" for financial gain by elements of the food industry - raising wider concerns about the safety of the contaminated products.
"It seems improbable that individuals prepared to pass horsemeat off as beef illegally are applying the high hygiene standards rightly required in the food production industry," it said.
"We recommend that the Government and FSA undertake a broader spectrum of testing for products found to have the highest levels of contamination ... to provide assurances that there is no other non-bovine DNA or any other substances that could be harmful to human health present."
Meanwhile, a processing factory in Co Monaghan in the Republic of Ireland withdrew some batches of burger products produced for the UK market, some of which tested positive in the UK for 5% to 30% horse meat.
Rangeland Foods said the burgers, which contained beef supplied from Poland, date back to production in September and were specifically produced for the UK market and made to a specification for EU beef from EU-approved suppliers.