The Food Standards Agency has raided a slaughterhouse allegedly involved in supplying horsemeat labelled as beef.
The FSA has shut down Peter Boddy Licensed Slaughterhouse in Todmorden, West Yorkshire, while it investigates allegations that it supplied horse carcasses to a meat business in Wales.
Officers from West Yorkshire and Dyfed-Powys police accompanied the FSA as they seized meat and paperwork from the Yorkshire abattoir and Farmbox Meats Ltd in Llandre, Aberystwyth.
The FSA and police are looking into the circumstances through which meat products, purporting to be beef for kebabs and burgers, were sold, which were in fact horse.
The FSA has suspended operations at both sites while it investigates the first suspected instance of a UK abattoir passing off horsemeat as beef.
Meanwhile, Waitrose has announced it is clearing the shelves of its Essential British Frozen Beef Meatballs after pork was detected in tests on two batches.
Environment Secretary Owen Paterson said he expects tough action to be taken against any business that has broken the law.
He 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."
Slaughterhouse owner Peter Boddy, who denied that the FSA visit amounted to a "raid", told Sky News: "I have not been supplying meat to Farmbox. I don't know who they are."
Farmbox Meats Ltd has also denied any wrongdoing.
In an interview with Sky News, FSA Director of Operations Andrew Rhodes said: "We acted on excellent evidence, which includes the traceability of where products go from one location to another.
"I'm very confident that the information we have used and what we have obtained is evidence that something has happened which should not have been."
The raids came after a respected food scientist warned that lamb ready meals and other products may contain horsemeat and should be tested.
Mr Paterson today met representatives of supermarkets and food suppliers to discuss the growing scandal of horse meat mislabelled as beef.
Joining officials from the Food Standards Agency, he talked to the Institute of Grocery Distribution, which represents food retailers and suppliers, to discuss plans for a new regime of quarterly testing of products.
Results of tests into the extent of contamination of beef products are expected on Friday.
The Environment Secretary will travel to Brussels tomorrow to discuss the scandal with counterparts in EU countries.