Testing on Findus beef lasagne has revealed the meals contain more than 60% horse meat, the UK's Food Standards Agency has said.
Anyone who bought the products is advised not to eat them.
Findus withdrew the beef lasagne products after its French supplier, Comigel, raised concerns about the type of meat used in the product.
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 it had no evidence to suggest it was a food safety risk, but ordered the company to test the lasagne for the veterinary drug phenylbutazone, or "bute".
Animals treated with that drug are not allowed to enter the food chain as it may pose a risk to human health.
The Findus beef lasagne was distributed to the main UK supermarkets and smaller convenience stores.
The Irish Food Safety Authority said it had not been made aware if the product was on sale in Ireland.
However, the FSAI has advised consumers who may have purchased the product not to consume it.
Findus has already begun a full recall of the products.