Phenylbutazone is an anti-inflammatory treatment for horses which is potentially harmful to humans and by law is supposed to be kept off of plates.
Britain alerted Paris that six tainted carcasses had been exported to France in January, but the meat had already been processed by the time the warning came, the spokesman said.
Although some of the meat had been recalled, the equivalent of three carcasses had "probably" made it to consumers, he said, adding that there was only a "minor" health risk.
The announcement added a new dimension to the scandal over mislabelled meat that erupted in Europe in January after horsemeat was initially found in so-called beef ready-made meals and burgers in Britain and Ireland.
Since then, supermarkets across the continent have pulled prepared meals from their shelves, with effects felt away as Hong Kong, where an imported brand of lasagne has been withdrawn from stores.
French President Francois Hollande said on Saturday that he would push for mandatory labelling of meat in ready-made meals.
"I want there to eventually be mandatory labels on the meat contained in prepared meals," Francois Hollande said while visiting an agricultural show in Paris.
"Until then, I will support... all initiatives for voluntary labelling" so that "consumers know the origin of the products they are consuming, especially meat."
French firm Spanghero has been at the heart of the scandal after it allegedly passed off 750 tonnes of horsemeat as beef, with the product eventually finding its way into 4.5 million "beef" products sold across Europe.
French authorities had initially suspended the company's sanitary license, but following protests from 300-odd workers allowed the company to resume production of minced meat, sausages and ready-to-eat meals.
The company was banned, however, from stocking frozen meat.
In Ireland, authorities on Friday suspended production at a meat processing plant after investigators found it was selling horsemeat labelled as beef.
B&F Meats, a small company licenced to debone beef and horsemeat in Carrick-on-Suir in County Tipperary, was found to be sending horsemeat to a customer in the Czech Republic, the Irish agriculture ministry said in a statement.
The label in the Czech language refers to beef, it added.