Food companies will be ordered to test their beef products after some Findus beef lasagnes were found to contain up to 100% horse meat, the Food Standards Agency (FSA) said.
It was "highly likely" that criminal activity was to blame for the contamination, the agency added, as consumers were warned not to eat the meals.
Findus UK tested 18 of its beef lasagne products and found 11 contained in the range of 60% to 100% horse meat, the FSA said.
The frozen food company apologised to customers and said refunds would offered to anyone who bought the affected lasagne products, which were made by French food supplier Comigel.
Retail giant Tesco and discount chain Aldi have withdrawn a range of ready meals produced by Comigel over fears that they contained contaminated meat.
Catherine Brown, chief executive of the FSA, told BBC News: "This is an appalling situation. I have to say that that the two cases of gross contamination that we see here indicates that it is highly likely there has been criminal and fraudulent activity involved."
She added: "We are demanding that food businesses conduct authenticity tests on all beef products, such as beef burgers, meatballs and lasagne, and provide the results to the FSA. The tests will be for the presence of significant levels of horse meat."
There is no evidence to suggest the horse meat found is a food safety risk, the FSA said. But the agency confirmed tests have been ordered on the lasagne for the veterinary drug phenylbutazone or "bute", which is banned from entering the food chain.
A Findus UK spokesman said: "We understand this it is a very sensitive subject for consumers and we would like to reassure you we have reacted immediately. We do not believe this to be a food safety issue. We are confident that we have fully resolved this supply chain issue. Fully compliant beef lasagne will be in stores again soon.
Findus UK withdrew its 320g, 360g and 500g lasagne meals from supermarket shelves as a precautionary measure earlier this week. It came after Comigel alerted Findus and Aldi that their products "do not conform to specification".