* Processed food market increasingly complex
* Consumers worried about little-regulated industry
* Britain: "This is a conspiracy"
* Lawmaker calls for ban on meat imports from EU
By Tim Castle and Maria Golovnina
LONDON, Feb 10 (Reuters) - As Britons choke on discovering they may have eaten horse that was imported as beef, and ministers blame an "international criminal conspiracy", this new scandal has exposed the sometimes murky labyrinth by which food reaches Europe's dinner tables.
Lurid headlines reveal a culinary gulf between distaste for the notion of horsemeat in Britain and its status as a delicacy elsewhere in Europe. But as governments play down the health risks, a greater impact may stem from a shattering of public confidence in EU systems of labelling and quality control introduced after previous threats hit the human food chain.
As details emerge of a complex network of slaughterhouses and middlemen standing between the farm and the supermarkets across Europe, France and Britain have vowed to punish those found responsible for selling horsemeat purporting to be beef.
With DNA tests needed to tell the two kinds of flesh apart, retailers and makers of processed meals complain of being duped by suppliers; one French firm has pointed a finger at Romania.
"This is a conspiracy against the public," said British farm minister Owen Paterson. "I've got an increasing feeling that it is actually a case of an international criminal conspiracy."
Prime Minister David Cameron has called it "very shocking".
Adding to concerns are indications that some horsemeat, perfectly edible in itself, may contain a drug known as bute - a common, anti-inflammatory painkiller for sporting horses but banned for animals intended for eventual human consumption.
Britain's Food Standards Agency said it was checking whether horse carcasses exported from Britain contained phenylbutazone. It said five such animals were sold abroad last year and it had told foreign agencies. French media said the horses went there.
One firm hit by the British horsemeat scandal, frozen foods group Findus, said it was recalling its beef lasagne product after discovering they included horsemeat. Its French supplier, Comigel, said the questionable meat came from EU member Romania.
An EU-wide alert has been sent out and governments debated how to bring the increasingly complex industry under control.
Food experts say globalisation has brought benefits to food supply, with exotic items now available from around the world all year round, but it has also created a system that is so complex it has increased the risks of adulteration, whether by design, to use cheaper inputs, or through neglect of standards.
The "mad cow" crisis, which saw British beef banned in the EU in the 1990s over fears of a degenerative brain disease, left a legacy of tight controls on the identity of European animals, intended to ensure the origins of fresh meat are traceable.
But in meat minced into processed product, while hygiene checks are the norm, testing for something as seemingly basic as which species it came from is complex and not widely undertaken.
Mystery over the contents of a sausage is far from new, but mass production means any problem can escalate rapidly:
"Food adulteration has been going on for as long as it has been prepared, for thousands of years," said Chris Elliot, a professor working on food safety at Queen's University Belfast.
"We are at the stage now where whenever this adulteration happens, it tends to happen on a very large scale, extremely well organised."
Doubts over quality controls in processed food could damage sales across Europe, but the greatest impact of this scandal may be in Britain, where assurances that horsemeat is safe have done little to lessen the disgust felt by many, or suspicions that it reflects another unpopular aspect of membership of the EU bloc.
One leading British lawmaker called for a ban on EU imports.
"Nabbed, stabbed and beaten: wild horses to go in our beef," ran the headline on Sunday's mass-selling Sun newspaper over a story alleging cruelty to horses to be slaughtered in Romania.
From Queen Elizabeth downwards, Britons cast themselves as a nation of horse lovers, treating sporting thoroughbreds with no less reverence than human athletes and viewing the species as whole with an affection rivaled only by that for the family dog.
There are only a handful of licensed horse abattoirs in Britain, and these mostly export carcasses to the continent, where Italy leads consumption tables with an unsentimental taste for both horse and donkey; horsemeat also has a niche in the cuisine of France and of many other European nations.
At a Sunday market in north London, where shoppers strolled among rain-soaked stalls selling vegetables, sausage and cheeses direct from the small farms that produce them, many said they would buy fewer frozen ready-meals after the revelations.
That is good news for the likes of Amie Peters, who runs a family beef burger business: "They've kept it secret from everyone. It was concealed from the public. That's not nice," she said of equine DNA found in supermarket burgers, nodding to her own grill and adding with a smile: "No horsemeat in these."
Distaste for horsemeat is widely shared across the English-speaking world, although the U.S. Congress in 2011 overturned a five-year-old effective ban on slaughtering horses for food.
Tracing processed meat back to its source is difficult in Europe's complex market, and the path from abattoirs where cows and horses are slaughtered and minced to people's dinner tables often meanders through a confusing chain of middle companies.
Last week's problems for Findus came less than a month after British supermarket Tesco and fast food outlet Burger King found horsemeat in beef burgers from Ireland.
French officials tracing the contamination of the Findus beef lasagne said a Luxembourg factory had been supplied by the French firm Poujol, which had bought the meat frozen from a Cypriot trader, who in turn had subcontracted the order to a Dutch trader supplied by a Romanian abattoir.
However, Comigel, a frozen foods producer based in eastern France, told a newspaper it had bought the meat from another French company, supplied from a Romanian abattoir.
In Romania, officials said one of the two Romanian abattoirs suspected to have provided horsemeat had been cleared of all suspicion: "I believe that, even though the investigation isn't finished, that everything left the country properly and officially," Constantin Savu of Romania's food safety authority was quoted as saying by state news agency Agerpres on Sunday.
"I find it hard to believe that such errors could exist."
In France, six big retailers said they were recalling lasagne and other products suspected to be mislabelled.
Britain's Paterson summoned leading food retailers and representatives of food processors to an emergency meeting at his office at the weekend to discuss the crisis.
Anne McIntosh, who chairs the parliamentary food and environment committee, called for a temporary import ban on processed and frozen meats from the other 26 EU states.
"My concern is that consumer confidence will have collapsed across the European Union," McIntosh, from Cameron's Conservative party, told the BBC on Sunday.
"We seem to be no clearer as to what the source of this contamination is, or whether the supply was ever destined for human consumption. Is this a fraud of such a massive scale that it should never have entered the human food chains?" (Additional reporting by Luiza Ilie in Bucharest and Leigh Thomas in Paris; Writing by Maria Golovnina; Editing by Alastair Macdonald)