Food retailers have been told to carry out tests on all processed beef products after some Findus lasagnes were found to contain 100 per cent horsemeat.
The Food Standards Agency (FSA), which ordered the UK tests, said it was “highly likely” criminal activity was to blame for the contamination.
It said there was no evidence of a health risk, but its chief executive said it was an “appalling situation”.
Findus has apologised to customers and withdrawn the meals from sale.
Findus is the latest company to be caught up in the controversy surrounding contamination of meat products, which has affected companies in the UK, Irish Republic, Poland and France.
Findus’s affected products were made by a third-party French supplier, which had alerted the company to concerns that the beef lasagne product did not “conform to specification”.
Catherine Brown, the FSA’s chief executive, said: “I have to say 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.
“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 horsemeat.”
The FSA has ordered Findus to test the contaminated lasagne for the veterinary drug phenylbutazone, or “bute”.
“Animals treated with phenylbutazone are not allowed to enter the food chain as [the drug] may pose a risk to human health,” it said.
Findus had withdrawn its beef lasagne in 320g, 360g and 500g sizes as a precaution on Monday.
The FSA said Findus had tested 18 of its beef lasagne products and found 11 containing between 60 and 100 per cent horsemeat.