Test results for horsemeat in British processed meals are due today - as detectives continue questioning three men over the scandal.
The Food Standards Agency (FSA) will report on UK products after asking retailers and suppliers to provide "meaningful results" from tests to detect the presence of horsemeat in processed meals labelled as beef.
The FSA said it wanted the food industry to show that the food it sells and serves is what it says it is on the label.
The test results, for significant levels of horsemeat, 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.
"It is not acceptable for retailers to remain silent while their customers have been misled," a senior Downing Street source told The Daily Telegraph. "The supermarkets need to justify their action and reassure the public."
The eagerly-awaited test results will emerge as police in Wales question 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 he 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, at 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 horsemeat for beef.
Production at both plants was suspended pending the outcome of investigations into claims that 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.
On Thursday it emerged that a significant amount of horsemeat 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 horsemeat is commonly eaten.
Announcing the results of the bute tests in the House of Commons, agriculture minister David Heath said the Government had instigated the "biggest investigation ever" into criminal activity in Europe over horsemeat 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 horsemeat 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 horsemeat found no evidence of the substance.
Chief Medical Officer Dame Sally Davies played down 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.
Ms 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."