Tests confirming lasagne sold under the Findus brand contained up to 100 percent horsemeat sparked a wider food scare in Britain on Friday, with authorities ordering urgent tests on all beef products on sale.
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.
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.
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FSA director of operations Andrew Rhodes said there were probably two causes.
"The first is gross negligence and the second may be criminal activity, and that's what we are investigating at the moment — whether somebody has deliberately substituted one product for another," he told BBC radio.
Rhodes 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," he said.
"We are demanding that all the manufacturers, all the retailers, test all of their products to rule out any further contamination.
"It's the responsibility of those making food and selling food to ensure that what's in that food is exactly what it says on the label.
"I can't speculate on what we might find in further testing, but if we do find something then we will be dealing with that very robustly."
FSA chief executive Catherine Brown told the BBC it was "highly likely there has been criminal and fraudulent activity involved."
The FSA said it had ordered further tests on the suspect lasagne for the veterinary drug phenylbutazone, as animals treated with it are not allowed to enter the food chain in Britain.
It is the latest horsemeat-related scare after equine DNA was found two weeks ago in beefburgers in Britain and Ireland, countries where horse meat consumption is generally taboo.
Millions of beefburgers have been removed from sale.
Will the lasagne news tip the scales of public concern? As GlobalPost's Barry Neild reported last month, citizens of the UK appeared none too concerned about what they were eating for dinner. "Politicians, journalists and the general public seem to have swapped more jokes than concerns about the quality of food," Neild wrote.
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 asked all suppliers, including Comigel, in light of recent issues with meat, to provide certification about exactly what meat was in their meat products," 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 until tests were carried out, which subsequently showed that there was horsemeat in some samples.
"We don't believe it's a health safety issue, but obviously 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 and in central Asia, China and Latin America.
Comigel was not immediately able to respond when contacted by AFP.
Two weeks ago, the Food Safety Authority of Ireland had revealed that up to 29 percent of the meat content of some beefburgers was in fact horse, while they also found pig DNA.
The frozen burgers with horse DNA traces were on sale in high-street supermarket chains Tesco and Iceland in both Britain and Ireland, and in Irish branches of Lidl, Aldi and Dunnes Stores.
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.
GlobalPost's Barry Neild contributed reporting from London.