David Cameron has promised that anyone involved in passing off horse meat as beef would face the full force of the law after two British plants were raided and shut down.
The Prime Minister defended the Government's response to the growing scandal, saying it was insisting on "meaningful" tests of products by retailers and suppliers.
"If there has been criminal activity there should be the full intervention of the law," he said.
The comments came after the Peter Boddy slaughterhouse in Todmorden, West Yorkshire, and meat processing plant Farmbox Meats at Llandre in Aberystwyth, west Wales, had records seized by the Food Standards Agency (FSA) and were temporarily shut down amid claims they supplied and used horse carcasses in burgers and kebabs.
Mr Cameron said it was "appalling" and "completely unacceptable" that consumers were buying beef products that turned out to contain horsemeat.
He said many of the current issues had come to light due to tougher tests that had been ordered by ministers, and pledged that in future results would be made public.
Earlier, FSA director of operations Andrew Rhodes said its probe had discovered "blatant misleading of consumers" and would continue until there was "nothing left to find". He told BBC Breakfast: "Our investigations have determined that we found horsemeat that was produced at the plant in West Yorkshire being sent to a location in west Wales, a business called Farmbox."
"We entered that premises yesterday, FSA enforcement officers entered that premises, following up on the evidence that we found and we spoke to the staff there and we seized product, and we found that horsemeat had been used as though it were beef in kebabs and burgers at that premises."
Asked if he was alleging that horsemeat was passed off as beef, Mr Rhodes said: "That is absolutely right, yes. The Food Standards Agency is a science and evidence-led organisation and we don't do things lightly. We have very good evidence to support the actions that we have taken."
He said five slaughterhouses in the UK process horses on a regular basis. "What we have been doing is investigating the trail from all of them," he said. "As I say, the majority of everything we have found has been completely in order as we would expect it to be."