Tests confirming that a brand of lasagne contained up to 100 percent horsemeat sparked a wider food scare in Britain on Friday, with authorities ordering urgent tests on all beef products.
The Food Standards Agency said criminal activity was likely to blame as consumers grappled with an escalating horsemeat scandal on the shelves of major British supermarkets.
Food company Findus tested 18 of its beef lasagne products manufactured by French supplier Comigel and found 11 meals containing between 60 percent and 100 percent horsemeat, the FSA said.
FSA director of operations Andrew Rhodes said either gross negligence or criminal activity was to blame.
He stressed there was no evidence that the products posed a risk to public health.
"We are testing a very broad range of products, including those that go to schools and hospitals," Rhodes told BBC radio.
"We are demanding that all the manufacturers, all the retailers, test all of their products to rule out any further contamination.
"I can't speculate on what we might find in further testing."
FSA chief executive Catherine Brown told the BBC it was "highly likely there has been criminal and fraudulent activity involved".
The agency said it had ordered further tests on the suspect lasagne for the veterinary drug phenylbutazone, which can cause a serious blood disorder to humans in rare cases.
It is the latest horsemeat-related scare after equine DNA was found two weeks ago in beefburgers in Britain and Ireland, countries where horsemeat consumption is generally taboo.
Millions of beefburgers have been removed from sale.
Prime Minister David Cameron's Downing Street office described the latest incident as "distasteful" but stressed there was no evidence of a health risk.
A spokeswoman said the matter appeared to involve "acts of criminality" which were being investigated by the police, including officers from Britain.
Agriculture and Food Minister David Heath urged consumers to continue with their usual shopping habits until told to do otherwise.
He told BBC radio he was "staggered" by the findings so far.
"I cannot honestly say at the moment that the content of every burger is as it should be because we haven't got the evidence to do so," he added.
A spokesman for Sweden-based Findus's operations in Britain told AFP that its lasagne products were made by Comigel in Luxembourg. The French firm has supplied beef lasagne for Findus since 2011.
He said the lasagnes were sold in Tesco, ASDA and Morrisons, three of Britain's so-called "big four" supermarket chains.
Findus has asked all its suppliers to provide certification about exactly what meat is in their meat products since the horsemeat scandal first broke, the spokesman said.
"Last weekend, Comigel said they were not sure about the lasagne," he said, adding that the product was withdrawn on Monday in Britain while tests were carried out, which subsequently revealed horsemeat in some samples.
"It's something that is being investigated and its not something pleasant especially for UK customers who feel very sensitive about eating horse."
The consumption of horsemeat is more common in parts of Europe including France, as well as in central Asia, China and Latin America.
Comigel said Friday there was no risk from the horsemeat.
Based in the northeastern French city of Metz, Comigel employs 200 people and supplies frozen meals to supermarket chains and other clients in 15 countries, with Germany, the Netherlands, Belgium and Scandinavia the main markets, according to industry websites.
Two weeks ago, the Food Safety Authority of Ireland had revealed that up to 29 percent of the meat content of some frozen beefburgers was in fact horse, while they also found pig DNA. They were on sale in major supermarket chains.
In a separate scare last week, Britain suspended a company supplying halal meat to prisons after traces of pork were found in meat pies and pasties.