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".