Three men were arrested last night by officers investigating the horse meat scandal.

The men were arrested at plants in Aberystwyth and Todmorden, West Yorkshire, which were inspected on Tuesday by the Food Standards Agency.

A Dyfed-Powys Police spokesman said: "Dyfed- Powys Police have made arrests at both meat plants inspected by the Food Standards Agency on Tuesday.

"At Farmbox Meats near Aberystwyth, Dyfed-Powys, police have arrested two men aged 64 years and 42 years, and in a simultaneous operation police arrested a man aged 63 at the Peter Boddy Slaughterhouse in Todmorden, West Yorkshire.

"Approvals for both operations were suspended yesterday by the FSA so neither firm was operational. Dyfed Powys Police can confirm the three people have been arrested on suspicion of offences under the Fraud Act."

The FSA has said a significant amount of horsemeat containing the painkiller bute could have been entering the food chain for some time. The watchdog is preparing to announce results of tests on products sold in the UK.

Authorities in Britain and France are trying to trace the carcasses of six horses contaminated with phenylbutazone – or "bute" – which were slaughtered in a UK abattoir and may have entered the human food chain in France.

The drug was detected in eight horses out of 206 tested by the FSA this month.

FSA chief executive Catherine Brown said the agency increased horse carcass testing last year. "That would say there has been a significant amount of carcasses with bute in going into the food chain for some time," she said.

Chief Medical Officer for Scotland, Sir Harry Burns said: "The positive samples show minute traces of Phenylbutazone compared to the normal dose which used to be given to humans. Even the sample with the highest trace is still at a level many hundred times lower than those previously used in humans on a daily basis."

"Furthermore, even at the much higher level previously given to humans, serious adverse reactions were rare and usually associated with prolonged exposure.

"It is, therefore, my view that the risk to human health is likely to be very low. However, no level of this drug in the human food chain can be tolerated. I am, therefore, reassured by the action the FSA is taking to ensure this is the case."