A plant in West Wales and a slaughterhouse in Yorkshire are under investigation in connection with allegedly supplying horse meat mislabelled as beef.
Peter Boddy Licensed Slaughterhouse, in Todmorden, is believed to have supplied horse carcasses to Farmbox Meats Ltd, Llandre in Aberystwyth.
Both plants were suspended yesterday while investigations continue.
The Food Standards Agency and the police are looking into the circumstances through which meat products, allegedly mislabelled as beef for kebabs and burgers, were sold.
A police spokesman said: "Dyfed-Powys Police accompanied the Food Standards Agency officials to the meat premises in Llandre, Aberystwyth and facilitated their entry into the premises. At this time it is an FSA investigation."
Andrew Rhodes, FSA Director of Operations, ordered an audit of all horse producing abattoirs in the UK after the issue first arose last month.
Mr Rhodes said he was shocked to uncover what appears to be "blatant misleading of consumers."
Environment Secretary Owen Paterson said: "This is absolutely shocking. It’s totally unacceptable if any business in the UK is defrauding the public by passing off horsemeat as beef.
"I expect the full force of the law to be brought down on anyone involved in this kind of activity."