In addition to the arrests, French investigators believe a processing plant involved in Europe's horse-beef scandal suspected horsemeat was mislabeled, adding to recent concerns that horse drugs may have entered the human food chain through its sales.
British police say three men have been arrested by officers investigating the burgeoning horse-meat scandal in Europe.
Police in Wales said Thursday's arrests on suspicion of fraud offenses occurred at two plants that were inspected earlier this week by the U.K.'s Food Standards Agency.
Police said two men, ages 64 and 42, were arrested at Farmbox Meats near Aberystwyth, in Wales, while a 63-year-old man was arrested at the Peter Boddy Slaughterhouse in Todmorden, West Yorkshire.
The arrests come after Britain's food regulator said six horse carcasses that tested positive for an equine painkiller may have entered the human food chain in France and that horse meat tainted with the medicine may have been sold to consumers "for some time."
Also on Thursday, a French investigation into a meat-labeling scandal has identified the French meat-processing company Spanghero as a likely culprit in the horsemeat scandal that has enraged consumers across Europe and implicated traders and abattoirs from Cyprus to Romania. The French probe found that Spangero sold horsemeat labeled as beef, French Consumer Affairs Minister Benoit Hamon said Thursday, a claim the company denied.
"The investigation shows that Spanghero knew that the meat labeled as beef could be horse. There was a strong suspicion," Hamon told a news conference, adding that a Romanian supplier named in the affair seemed to have been acting in good faith.
Spanghero rejected accusations that it knowingly sold horsemeat labeled as beef and said it believed that it was selling was beef.
"Spanghero confirms having placed an order for beef, having been led to believe it received beef, and having sold back what it thought was beef, properly labeled as such, in line with European and French regulations," the firm said in an emailed statement.
Agriculture Minister Stephane Le Foll said the government was considering withdrawing Spanghero's operating license.
Earlier Thursday, Britain's Food Standards Agency said six horses slaughtered in the UK that tested positive for the drug phenylbutazone were exported to France and may have entered the human food chain.
Phenylbutazone, commonly known as bute, is an anti-inflammatory painkiller for sporting horses but banned for animals intended for eventual human consumption as it is potentially harmful.
Britain's food regulator said Thursday it was gathering information on the six carcasses sent to France and will work with the French authorities to trace them.
The FSA said it checked 206 horse carcasses between Jan. 30 and Feb. 7. Of these, eight tested positive for the drug.
It said the six sent to France were slaughtered by LJ Potter Partners at Stillman's (Somerset) Ltd. in Taunton, western England.
The remaining two did not leave the slaughterhouse in the U.K. — High Peak Meat Exports Ltd., Nantwich, northwest England — and have now been disposed of.
The FSA introduced 100 percent testing of horse carcasses Jan. 30 in response to the growing horsemeat scandal.
The issue first came to light Jan. 15 when routine tests by Irish authorities discovered horsemeat in beef burgers made by firms in Ireland and Britain and sold in supermarket chains, including Tesco, Britain's biggest retailer.
Concern grew last week when the British unit of frozen foods group Findus began recalling its beef lasagna on advice from its French supplier, Comigel, after tests showed concentrations of horsemeat ranging from 60 to 100 percent.
The revelations, affecting a growing number of countries, processors and retailers, have raised uncomfortable questions about the safety of the European food supply chain and prompted governments to send out a European Union-wide alert.
Earlier Thursday a British parliamentary report into the scandal said discoveries so far of horsemeat in products sold as beef are likely to be the "tip of the iceberg."
Reporting by James Davey and Sybille de la Hamaide