Test results for horse meat in British processed meals are due on Friday as detectives continue questioning three men arrested in connection with the mis-labelling scandal.
The Food Standards Agency (FSA) will report on the UK products after asking retailers and suppliers to provide "meaningful results" from tests to detect the presence of horse meat in processed meals labelled as beef.
Test results on British foods are due Friday lunchtime
The FSA said it wanted the food industry to show the food it sells and serves is what it says it is on the label.
The test results, for significant levels of horse meat, will come from all beef products such as burgers, meatballs and lasagne.
Prime Minister David Cameron reportedly believes supermarkets have been too silent on the scandal.
A senior No10 source told the Daily Telegraph: "It is not acceptable for retailers to remain silent while their customers have been misled. The supermarkets need to justify their action and reassure the public."
The eagerly-awaited test results will emerge as police in Wales probe three men arrested on suspicion of offences under the Fraud Act.
The trio were taken into custody from two plants inspected and temporarily shut down by the FSA on Tuesday.
Sources said Dafydd Raw-Rees, 64, owner of Farmbox Meats near Aberystwyth, was arrested along with a 42-year-old man.
A 63-year-old man was also arrested on suspicion of the same offence at Peter Boddy Slaughterhouse in Todmorden, West Yorkshire.
On Wednesday, owner Mr Boddy, 63, said he had done nothing wrong and insisted the FSA inspection was merely to look at his records, but was last night unavailable for comment.
His firm is also contracted to remove fatally injured horses from the Grand National.
It removes the carcasses of some horses which have been put down during the world famous meeting, Aintree Racecourse, and said it was "confident" no unfit meat had entered the food chain.
The arrests were made by Dyfed-Powys Police in a joint operation with the FSA.
The two plants became the first UK suppliers suspected of passing off horse meat for beef.
Production at both plants was suspended pending the outcome of investigations into claims they supplied and used horse carcasses in meat products purporting to be beef for burgers and kebabs.
The FSA said on Tuesday it had "detained" all meat found at the premises and seized paperwork and customer lists from the two companies.
Last night's arrests were made as Asda withdrew its 500g beef bolognese sauce from shelves after tests revealed the presence of horse DNA, the supermarket chain said.
The company apologised to customers and said it was taking a "belt-and-braces approach" by removing a further three beef products made by the same supplier, the Greencore plant in Bristol, as a precaution.
Yesterday it emerged a significant amount of horse meat containing the painkiller phenylbutazone - or "bute" - could have been entering the food chain for some time.
Authorities in Britain and France are trying to trace the carcasses of six horses contaminated with bute - which were slaughtered in a UK abattoir and may have entered the human food chain across the Channel.
The drug, which is potentially harmful to human health, was detected in eight horses out of 206 tested by the FSA in the first week of this month.
Two were intercepted and destroyed before leaving the slaughterhouse but the other six were sent to France, where horse meat is commonly eaten.
Announcing the results of the bute tests in the House of Commons yesterday, agriculture minister David Heath said the Government had instigated the "biggest investigation ever" into criminal activity in Europe over horse meat contamination of beef products.
FSA chief executive Catherine Brown said the agency increased testing of horse carcasses over a three-month period last year after intelligence from abattoirs suggested bute was getting into the food chain.
Of 63 tested - amounting to 5% of all carcasses - four (6%) tested positive for the painkiller, prompting the FSA to start testing 100% of horse meat in January, which revealed the eight contaminated carcasses.
Ms Brown said: "That would say there has been a significant amount of carcasses with bute going into the food chain for some time."
However tests on Findus processed beef products withdrawn from sale in the UK after the discovery of traces of horse meat found no evidence of the substance.
Chief Medical Officer Dame Sally Davies downplayed the risk saying: "If you ate 100% horse burgers of 250g, you would have to eat, in one day, more than 500 or 600 to get to a human dose."
The highest level of bute found in tests was 1.9 milligrammes per kilo of meat.
Brown said both vets and horse owners have to sign horse passports if an animal is treated with bute, to ensure it is not subsequently sold on for human consumption.
"If both these people have done the right thing, horses with bute in don't make their way into the food chain," she said.
"Someone has always broken the rules."
Helen Dickinson, director general of the British Retail Consortium, which represents supermarkets, rejected claims that its members were slow to remove products potentially containing horse meat from their shelves.
"Retailers have not been reticent, they have been working hard over the last three weeks since this issue first arose to make sure any problems they do identify they act on quickly," she told BBC News.
"And we have seen them do that, withdrawing products straight away from our shops."
She added: "Customers are rightly concerned and we are concerned too, and that is why we have been working really hard to identify any particular issues and withdraw those products as quickly as possible.
"The retail industry absolutely appreciates the role that it has, the trust that consumers place in it every day, and we need to ensure that we continue to work hard to restore that."
Duncan Campbell, a former president of the Association of Public Analysts and one of Britain's most senior food inspectors, said that today's test results would provide a "snapshot" of the food adulteration problem but warned: "I think there will be still more discoveries to be made."
Campbell told BBC Radio 4's Today programme: "The more people have looked for horse meat, the more products have been found containing it. I don't think we have got to the bottom of it yet.
"The food chain is very long and at any time there are ingredients and products progressing from the farm to the supermarket shelves. What we will have today is a snapshot of when these samples were taken, but it won't necessarily give us the whole picture just yet."
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