PARIS (REUTERS) - France said on Sunday managers at French firm Spanghero were responsible for passing off horse meat as beef, while Britain said it would investigate claims that warnings about horse meat entering the food chain were raised in 2011 but ignored.

Revelations that some beef dishes actually contained horse meat have caused a scandal across Europe, leading to products being removed from sale and police investigations. It has also cast a spotlight on food labelling and the complex supply chain across the European Union, damaging Europeans' confidence in the food on their plate and putting pressure on governments to explain lapses in quality control.

An investigation into activities at meat-processing firm Spanghero has revealed "serious, specific and coherent" reasons to suspect it knowingly defrauded customers and consumers by selling them horse meat labelled as beef, the government said.

However, the firm's 330 workers were not to blame and would meet with Agriculture Minister Stephane Le Foll on Monday to determine how they would be paid until the plant, whose sanitary licence has been revoked temporarily, could reopen.

"The government distinguishes responsibility for what appears to be the actions of Spanghero's leaders from the work of its employees," ministers for agriculture and consumer affairs said in a joint statement.

The scandal, affecting a growing number of European countries and retailers, began in Ireland when its food safety authority discovered horse meat in frozen beef burgers.

Investigations to determine how horse meat ended up in ready meals sold across Europe homed in on Spanghero, but further probes are under way targeting a meat trader in the Netherlands and other firms in the supply chain.

Britain's Sunday Times newspaper reported serious concerns had first been aired about horse meat with British food chiefs 18 months ago but nothing had been done.

Mr John Young, a former manager with Britain's Food Standards Agency (FSA), said he helped draft a letter to the government in April 2011 on behalf of Britain's biggest horse meat exporter, highlighting failures in the system to stop horses treated with the drug phenylbutazone from entering the human food chain.

"I discussed this with the chief executive of the Food Standards Agency this morning, and she is going to go back through the records and see exactly what was said at the time," Britain's Environment Secretary Owen Paterson told Sky News.

Phenylbutazone, commonly known as bute, is an anti-inflammatory painkiller for sporting horses but banned for animals intended for eventual human consumption as it is potentially harmful in large concentrations.

The FSA said last week six horses slaughtered in Britain had tested positive for bute and might have entered the human food chain.

Irish authorities first discovered horse meat in beef burgers made by firms in Ireland and Britain on Jan 15. However, the scandal widened two weeks ago when tests on beef lasagne made by the British unit of frozen foods group Findus showed the product contained up to 100 per cent horse meat.

Europol is now coordinating criminal investigations across Europe, and in Britain three premises have been probed, two closed down and number of arrests made, Mr Paterson said.

Spanghero had its licence revoked for the duration of the probe after French officials said it must have known it was buying horse meat, which is cheaper and carries a different customs code when delivered in frozen blocks.

The firm's president Barthelemy Aguerre struck back at accusations on Friday, saying the government had been too quick to point the finger.

But radio France Info reported that investigators had seized nearly 1,500 pages of invoices at Spanghero's factory in south-western France that showed the customs code "0205" for horse whereas the codes for beef are "0202" and "0201".

The radio cited unnamed investigators as saying Spanghero must have been aware of the difference between the codes, which are easily verifiable on a European Commission website.

Mr Paterson called for stricter food testing across Europe, and Austria's agriculture minister, Mr Nikolaus Berlakovich, called for European "food passports" that would let consumers see the origins of ingredients in processed foods.