A slaughterhouse in West Yorkshire has been raided by the Food Standards Agency after suspicions it supplied horse carcasses to be used in beef kebabs and burgers.
Peter Boddy Licensed Slaughterhouse in Todmorden has had operations suspended along with Farmbox Meats Ltd in Aberystwyth which received the carcasses.
In a statement the FSA said: "Both West Yorkshire and Dyfed-Powys police have entered the premises with the FSA. The FSA has detained all meat found and seized paperwork, including customer lists from the two companies.
Andrew Rhodes, FSA Director of Operations, said: "I ordered an audit of all horse producing abattoirs in the UK after this issue first arose last month and I was shocked to uncover what appears to be a blatant misleading of consumers.
"I have suspended both plants immediately while our investigations continue."
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."
And there have been fresh warnings over the contents of minced lamb products.
Ministers have ordered comprehensive tests on all beef products. Once these have finished, supermarkets will be told to test all other meats, the FSA said.
An FSA spokesman said: "We are right in the middle of this incident now, the urgency is around beef.
"Looking beyond this incident, it's important to ensure consumers have confidence in all products."
Meanwhile, a former head of the FSA has said some lamb products may contain horse meat.
Dr Mark Woolfe said dishes like lamb ready meals and doner kebabs should be tested immediately for horse DNA.