The Food Standards Agency (FSA) said eight horses slaughtered in the UK tested positive for the veterinary drug bute.
It said six of the animals were slaughtered by LJ Potter Partners at Stillman's (Somerset) Ltd in Taunton, Somerset. They were then sent to France and "may have entered the food chain".
The remaining two did not leave the slaughterhouse in the UK - High Peak Meat Exports Ltd in Nantwich - and have now been disposed of "in accordance with EU rules".
A total of 206 horse carcasses were tested between 30 January and 7 February.
"The FSA is gathering information on the six carcasses sent to France and will work with the French authorities to trace them", the agency added.