Testing on Findus beef lasagne has revealed the meals contain more than 60% horse meat, the Food Standards Agency (FSA) said.
Shoppers who bought the lasagne meals, which are produced by French food supplier Comigel on behalf of Findus, have now been warned not to eat them.
Retail giant Tesco and discount chain Aldi withdrew a range of ready meals produced by Comigel over fears they contained contaminated meat. The FSA said there is no evidence to suggest the horse meat found is a food safety risk.
But the agency confirmed tests have been ordered on the lasagne for the veterinary drug phenylbutazone. Animals treated with "bute" are not allowed to enter the food chain.
An FSA statement said: "As part of its ongoing investigation into mislabelled meat, the Food Standards Agency has confirmed that the meat content of beef lasagne products recalled by Findus has tested positive for more than 60% horse meat.
"Findus withdrew the beef lasagne products after its French supplier, Comigel, raised concerns about the type of meat used in the lasagne.
"We have no evidence to suggest that this is a food safety risk. However, the FSA has ordered Findus to test the lasagne for the veterinary drug phenylbutazone, or 'bute'. Animals treated with phenylbutazone are not allowed to enter the food chain as it may pose a risk to human health."
Findus has already started a full recall of its lasagne products, the FSA said. Anyone who bought the products should return them to the shop they were bought from, it added.
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".
It advised them to remove Findus beef lasagne and Aldi's Today's Special frozen beef lasagne and Today's Special frozen spaghetti bolognese. Tesco also decided to withdraw its Everyday Value spaghetti bolognese, which is produced at the same Comigel site.