LONDON (AFP) - Three men at meat plants in Britain suspected of passing horsemeat off as beef have been arrested on suspicion of fraud, police said on Thursday.

Two men were arrested in Aberystwyth on the Welsh west coast where a food processing plant is based and one was detained in West Yorkshire in northern England, where police raided a slaughterhouse on Tuesday.

Europe's scandal over horsemeat-tainted food spiralled on Thursday after Britain announced that a potentially harmful drug had been found in horse carcasses sent to France.

Britain's Food Standards Authority (FSA) said it had found traces of the equine drug phenylbutazone - which can cause a serious blood disorder in humans in rare cases - in six carcasses exported to France by British abattoirs.

"Six were sent to France and may have entered the food chain," the FSA said in a statement after testing the carcasses of 206 horses slaughtered in Britain.

"The FSA is gathering information on the six carcasses sent to France and will work with the French authorities to trace them."

British officials stressed that the low levels of the drug found in the meat posed little risk to human health.