The meat of some beef lasagne products recalled by Findus earlier this week was 100% horsemeat, the Food Standards Agency (FSA) has said.
On Monday Findus withdrew from retailers its beef lasagne in 320g, 360g and 500g sizes as a precaution.
Environment Secretary Owen Paterson said the findings were "completely unacceptable", but Findus said it did not believe it was a food safety issue.
The FSA said companies would now be required to test their beef products.
"In order to get to the bottom of this, we're going to be requiring every company to test every product line," Catherine Brown, the FSA's chief executive, told the BBC.
"If we find any other cases, we will pursue our investigations vigorously until we find out what's happened and put a stop to it."
Ms Brown said it was "highly likely" that criminal activity was to blame for horsemeat being found in some meals.
The FSA said Findus had tested 18 of its beef lasagne products and found 11 meals containing between 60% and 100% horsemeat.
People have been warned not to eat the products, which were made for Findus by French food supplier Comigel.