Eight horses, killed in the UK, tested positive for the painkiller bute and six may have entered the food chain in France, the Food Standards Agency said.
Some 206 carcasses were tested, with eight found to contain phenylbutazone.
England's chief medical officer said the highest level detected was 1.9mg of bute per kg of horsemeat, which posed "very little risk to human health".
Earlier, food minister David Heath said tests for traces of bute in Findus products were negative.
The horse carcasses were tested between 30 January and 7 February. The FSA said that, of the eight which tested positive for bute, six were slaughtered at LJ Potter Partners at Stillman's in Taunton, Somerset and were exported to France.
The two killed at High Peak Meat Exports in Nantwich, Cheshire, did not leave the slaughterhouse and have been destroyed.
The Prime Minister's spokesman told reporters that the UK was working very closely with the French authorities tracking the carcasses, which were identified on Thursday morning.
He 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."
During a briefing hosted by the Department for the Environment and Rural Affairs (Defra), chief medical officer Prof Dame Sally Davies said an individual would have to consume vast quantities of horsemeat containing bute to be at risk.
She said: "A person would have to eat 500 - 600, 100% horsemeat burgers a day to get close to consuming a human's daily dose. [The drug] passes through the system fairly quickly, so it is unlikely to build up in our bodies."
Bute is sometimes used as a drug to treat individuals suffering from a severe form of arthritis, but in rare cases it causes a serious blood disorder known as aplastic anaemia, where the body does not make enough new blood cells.
The FSA said they have implemented a positive release system across the UK, meaning horses slaughtered at British abattoirs must have tested negative for bute before they can enter the food chain.
FSA chief executive Catherine Brown accused some vets and horse owners of not ensuring horse passports are kept up-to-date, leading to bute-treated horses ending up in the food chain.
She said: "If both these people have done the right thing, horses with bute in don't make their way into the food chain. Someone has always broken the rules."
Responding earlier to an urgent question in the House of Commons, Mr Heath said that retailers and suppliers were "on course" to provide "meaningful results" on product-testing on Friday.
He confirmed that tests on Findus products had revealed no trace of bute. Findus withdrew its beef lasagne from sale after tests found it to contain up to 100% horsemeat.
Shadow environment secretary Mary Creagh - who tabled the Commons question - went on to accuse the government of complacency over the danger of bute entering the human food chain.
Ms Creagh first raised concerns in the House about bute contamination in January.
An earlier report from the Commons Environment, Food and Rural Affairs committee called for the FSA to have stronger powers to force meat producers to carry out testing.
Committee chairman Anne McIntosh told the BBC: "It is surprising to learn that [the FSA] can request testing by producers to be performed, but they don't currently have the powers to require testing to be performed. That would be one change that we would welcome."
In Ireland meanwhile, another meat processing factory has withdrawn some batches of burger products which contained beef supplied from Poland.
Rangeland Foods in County Monaghan said some of the food tested positive up to 30% horsemeat.
The products were sold to the catering and wholesale sectors in Ireland, Britain, Spain, France, Germany and the Netherlands, the Food Safety Authority of Ireland said.
Production at Rangeland has been allowed to continue under the proviso they use only Irish-sourced beef.