The British government accused unknown criminals for a growing scandal of horsemeat being sold in imported beef products that has generated shock headlines in a country where many recoil in horror at the very idea of eating horses.
Prime Minister David Cameron assured consumers on Friday there was no health risk from a product considered a delicacy in France and Italy. But, as the furore saps public confidence in food labelling and hygiene supervision, he called it "completely unacceptable", and his office condemned "acts of criminality".
Health officials said police had been called in.
Investigations into suppliers have been launched in recent weeks after the discovery that beef products sold to companies including Britain's biggest supermarket firm Tesco and fast-food chain Burger King contained horsemeat.
On Thursday, the scandal deepened further with the news that horsemeat had been found in Findus ready meals made in France, prompting the British government to call it "very distasteful" and forcing the firm to apologise to customers.
Some packs of "beef lasagne" may have contained no beef at all, only horse, officials said after genetic tests showed concentrations of horsemeat in a range from 60 to 100 percent.
"This is a very shocking story," Cameron said in Brussels where he was attending a European Union summit. "It is completely unacceptable.
"People will be very angry to find out that they have been eating horse when they thought they were eating beef."
The saga has offended many Britons' emotional self-image as a nation of animal lovers with a particularly soft spot for the horse and its place on the racecourse or in a vanished rural idyll. That their French neighbours consume it for lunch, is seen as no more palatable than their snails and frogs' legs.
More seriously, in the wake of health scandals including the "mad cow" disease which saw British beef exports banned for years by EU partners in 1996, the affair raises questions over the effectiveness of agencies supervising the food chain.
Cameron's spokeswoman said the government was looking into two incidents which "at the heart are acts of criminality" and Britain's Food Standards Agency (FSA) said it had asked police in London and in other European countries to investigate.
"If you are a company buying a particular meat and you are led to believe it is what you asked for, but then you find it's not, then clearly there's been some law broken there," Cameron's spokeswoman said.
French Agriculture Minister Stephane Le Foll said there would be an investigation there: "We need to avoid this idea that there was some desire to hide things," he told BFM television. "We will determine where the problem came from and we will find the solutions and sanctions that apply."
In a statement, Findus said it knew there was a possible problem with its ready meals two days before the products were withdrawn from the shelves of British stores.
"We understand those concerns, we are sorry that we have let people down and we want to outline the facts," it said.
"Findus UK had extensive DNA testing completed by leading independent experts. On Wednesday 6th February these tests confirmed that horsemeat was present in a number of samples and this information was shared with the Food Standards Agency."
Findus Sweden also said it had recalled thousands of packets of frozen "beef lasagne" after tests showed they contained horsemeat.
British supermarket chains Aldi, Lidl , Iceland and the Co-operative Group have also been sold beef products found to contain horse DNA.
The FSA put the horsemeat content at Findus at between 60 and 100 percent while one Tesco burger contained about 29 percent horsemeat, making those the two most serious incidents to date.
Tesco has since dropped the Irish supplier of frozen beef burgers, Silvercrest, a unit of ABP Food Group. Findus began a recall of its beef lasagne from retailers earlier in the week on advice from its French supplier, Comigel.
In response to the problem, the FSA has demanded that food retailers and suppliers test all beef products and present their findings to the agency by February 15.
Environment Minister Owen Paterson, who said "urgent" checks were being made on supplies to schools and hospitals, will also host a summit of meat retailers and suppliers on Saturday.
Two senior lawmakers advised on Friday against eating processed beef products, but Paterson said he would happily eat them and Cameron insisted there was no health risk.
"There is no reason to believe that any frozen food currently on sale is unsafe or a danger to health. It's not so much about food safety, it's about proper food labelling, it's about confidence in retailers," Cameron said.
Experts say horsemeat could contain traces of veterinary drug phenylbutazone, or "bute", used as a painkiller, which can be harmful to humans but only in high concentrations.
European plants processing horsemeat for food avoid carcasses in which the drug is present. French media last month reported complaints that horsemeat imported from Britain and contaminated with bute had been found in French food products.
However, the danger of eating such meat may be slight: "The idea that you might get a clinically significant amount in horsemeat, even after therapeutic administration to the horse is, frankly, daft," said Colin Berry, a professor of pathology at Queen Mary, University of London.
The FSA advised against eating Findus beef lasagne products, but - pending further testing for bute - said it had no evidence to suggest the product was a food safety risk.