Tests have shown that some Findus beef lasagne ready meals are 100% horsemeat, the Food Standards Agency (FSA) has revealed.
The products, made by the French company Comigel, have been taken off the shelves of convenience stores and supermarkets across the UK.
And the FSA has said the levels of horsemeat point to criminal activity rather than inadvertent contamination on the part of meat suppliers.
An FSA spokesman said: “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.”
Eight horses slaughtered for human consumption in the UK last year had been given bute.
People who have bought any Findus beef lasagne products are advised not to eat them and return them to the shop they bought them from.
Other companies that use meat from Comigel — including Aldi and Tesco — have removed beef products from their stores.
Ten million beef burgers were withdrawn from sale last month after traces horsemeat were found by the Food Safety Authority of Ireland.
World Horse Welfare has said the abuse of horses in the meat trade goes largely unreported.
Chief executive Roly Owers said: “Where there is horsemeat, you can bet there is horse suffering and not just at the time of slaughter.
“The whole European trade is mired in inadequate laws, needless suffering and the elephant in the room is the spread of infectious equine disease.”
He said the charity does not oppose humane slaughter or the eating of humanely produced horsemeat, which is a personal choice, but it is campaigning to stop the needless long distance transportation of 65,000 horses per year across Europe to slaughter.