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.
Retailers are preparing to say they have found no more traces of horse DNA in their beef products, but were enough products tested?
The strong taboos against eating horse in the UK are relatively new, and many cultures devour horses, saying they are healthy and tasty.