Two meat plants have been raided under suspicion of passing off horse meat as beef for kebabs and burgers.
The Food Standards Agency (FSA) and police officers entered Peter Boddy slaughterhouse in Todmorden, West Yorkshire, and Farmbox Meats in Llandre in Aberystwyth, West Wales.
They have both been suspended pending the outcome of investigations into claims they supplied and used horse carcasses in meat products purporting to be beef.
The FSA said it had "detained" all meat found at both premises and seized paperwork and 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 horse meat as beef. I expect the full force of the law to be brought down on anyone involved in this kind of activity."
The raids uncovered the first suspected instance of a UK abattoir passing off horse meat for beef, Mr Paterson confirmed to Sky News.
It came after the National Beef Association (NBA) suggested the addition of the words "United Kingdom origin" to packaging to prevent "further cheating" by suppliers on the Continent.
The scandal has spread all over Europe as details of the elaborate supply chain in the meat industry emerge. French consumer safety authorities said companies from Romania, Cyprus and the Netherlands, as well as its own firms, were involved.
The raids came as Waitrose announced it was pulling a range of beef meatballs after tests revealed they might contain pork. A spokesman said that tests on the 480g packs of 16 frozen Essential Waitrose Meatballs had been contradictory but it was removing them from sale as a precaution.