Waitrose announced on Tuesday it had withdrawn its beef Essential British Frozen Meatballs after pork was found in two batches.
It emerged today that they had been produced at the ABP Freshlink site in Glasgow last summer.
The affected batches were labelled Frozen Essential Waitrose Meatballs – 16 British Beef Meatballs 480g. One was marked Best Before End June 2013 and the other Best Before End August 2013.
Although there is no suggestion that horse was used in the meatballs, the revelation of the contents not matching the product forced Waitrose to act as the consumer backlash grows.
A Waitrose spokesman said: “Several tests have been done on this product and, even though the results have been contradictory, we have taken the precautionary action of removing the frozen meatballs from sale and putting up customer information notices in all our branches.”
It was announced in November last year that Freshlink in Shettleston, employing 150 people, was to close by April, and it is understood the factory is currently being wound up. The plans to relocate to ABP's plant in North Yorkshire prompted angry reaction across the political spectrum.
Responding to the claim that pork had been found in meatballs produced at its Glasgow site, an ABP spokesperson said: “Freshlink has carried out over 450 DNA tests during the last two and a half years. All our test results have been confirmed as negative for non-declared species.
“Freshlink will share all results directly with the Food Standards Agency (FSA) as part of their investigations.”
The FSA in Scotland said local safety inspectors from Glasgow City Council “regularly” visit the Freshlink factory to take samples “but the results have not yet been received”.
Meanwhile, Environment Secretary Owen Paterson will fly to Brussels today for a horse meat summit - after two other British processing plants were raided and shut down as part of the inquiry into the scandal.
The Food Standards Agency (FSA), accompanied by police, swooped on a slaughterhouse and a meat manufacturer after apparently uncovering "a blatant misleading of consumers".
Peter Boddy slaughterhouse in Todmorden, West Yorkshire, and meat processing plant Farmbox Meats at Llandre in Aberystwyth, West Wales, had records seized and were temporarily shut down pending the outcome of investigations into claims they supplied and used horse carcasses in burgers and kebabs.
The FSA said it had "detained" all meat found at the premises and seized paperwork and customer lists from the two companies.
Until now, meat linked to the scandal is thought to have come from suppliers in Europe, but for the first time it appears the contamination may also stem from British premises.
Andrew Rhodes, FSA director of operations, said: "I ordered an audit of all horse producing abattoirs in the UK after this issue first arose last month and I was shocked to uncover what appears to be a blatant misleading of consumers.
"I have suspended both plants immediately while our investigations continue."
David Cameron has now promised that anyone involved in passing off horsemeat as beef will face the full force of the law.
The Prime Minister defended the Government’s response to the growing scandal, saying it was insisting on ‘meaningful’ tests of products by retailers and suppliers.
He said: “If there has been criminal activity there should be the full intervention of the law.
“We have also asked for meaningful tests from retailers and producers and they will be published in full.”
Environment Secretary Owen Paterson will later travel to Brussels to discuss the scandal with counterparts in EU countries.
He said: "This is absolutely shocking. It's totally unacceptable if any business in the UK is defrauding the public by passing off horse meat as beef.
"I expect the full force of the law to be brought down on anyone involved in this kind of activity."
Slaughterhouse owner Peter Boddy said he would co-operate with FSA officers and claimed they had not "raided" his Todmorden premises.
He told ITV: "It was not a raid - they are welcome to visit whenever they want, they just wanted to see my records which I will be showing them."
The Welsh Government minister for agriculture, Alun Davies, said: "Integrity and trust are essential in the food chain.
"I would be appalled if these allegations are proven.
"The Welsh Government is working closely with the Department for the Environment, Food and Rural Affairs (Defra) and the FSA to ensure this matter is dealt with swiftly and decisively."
The raids came as Mr Paterson met representatives of the Institute of Grocery Distribution, which represents food retailers and suppliers, to discuss plans for a new regime of quarterly testing of products.
Results of tests into the extent of contamination of beef products are expected on Friday.
Supermarket giant Tesco yesterday became the latest retailer to drop a major supplier after discovering a range of spaghetti bolognese ready meals contained more than 60% horse meat.
It followed frozen food firm Findus and Aldi in finding the meat in products made by French firm Comigel and last night joined them in dropping the company as a supplier.
French consumer safety authorities said companies from Romania, Cyprus and the Netherlands, as well as its own firms, were involved.
Romanian authorities confirmed they are investigating while their Dutch counterparts said they are ready to do so if necessary.
But Romanian prime minister Victor Ponta has said his government had no evidence that any companies in Romania had broken any European laws.
The National Beef Association (NBA) has suggested the addition of the words "United Kingdom origin" to packaging to prevent "further cheating" by suppliers.
Asked if he believed there was a wider problem in the UK now, Mr Rhodes told BBC Breakfast: "We don't really have evidence to support that.
"What we have been doing at the FSA is investigating a really broad range of different things and we have actually exonerated quite a lot of businesses so far in our investigations and I am sure that will continue to be the case.
"What we are doing is focusing on the areas we think are the highest risk so we have identified documentary evidence that has led us to take the action that we have. We don't have evidence that this is a widespread problem in the UK."
He said five slaughterhouses in the UK process horses on a regular basis. "What we have been doing is investigating the trail from all of them," he said.
"As I say, the majority of everything we have found has been completely in order as we would expect it to be.
"We have got one particular business where we had some suspicions from the evidence we uncovered which we followed through and that led us to the investigations that we carried out near Aberystwyth yesterday.
"We will continue with that investigation and we will keep pursuing this until there is nothing left to find."
Mr Rhodes told BBC Breakfast: "Our investigations have determined that we found horse meat that was produced at the plant in West Yorkshire being sent to a location in West Wales, a business called Farmbox.
"We entered that premises yesterday ... enforcement officers entered that premises, following up on the evidence that we found and we spoke to the staff there and we seized product and we found that horse meat had been used as though it were beef in kebabs and burgers at that premises."
Asked if he was alleging that horse meat was passed off as beef, Mr Rhodes said: "That is absolutely right, yes.
"The Food Standards Agency is a science and evidence-led organisation and we don't do things lightly. We have very good evidence to support the actions that we have taken.
"Indeed we interviewed people on that site near Aberystwyth yesterday and those people confirmed that they received the goods that we believed they received and they told us what they had done with them.
"We are very clear on what has happened there.
"That is why we have seized all the meat that is there, that is why we have seized all the paperwork and that is why we involved the police both in Dyfed Powys and also in West Yorkshire."
Asked if he believed that meat purporting to be beef had reached retailers and therefore had been sold to consumers, Mr Rhodes told BBC Breakfast: "I think we have to use our language carefully.
"When we talk about retailers, people typically think about supermarkets. We don't have evidence of that at the moment, we also don't have evidence that this is being used in mass production."
Mr Rhodes said test results on processed beef products would begin to come in on Friday but full details could take much longer.
He told BBC Radio 4's Today programme: "Friday is when we have told the manufacturers and retailers to start submitting the highest risk areas. What they are looking for here is presence of horsemeat and pork in processed beef products.
"Obviously there are potentially hundreds of thousands of different products that need to be tested. They are testing at manufacturing level as well. So those results have to start coming through on Friday.
"We will get a very good number of those through on Friday and we will start to see what the picture looks like."
Asked if it could take months, Mr Rhodes said he could not say "exactly" but insisted the FSA was focusing on the highest risk products first.
Mr Rhodes denied claims that a surveillance programme had been effectively stopped previously under pressure from the supermarkets.
"We still actively sample lots and lots of products," he added.
Waitrose managing director Mark Price's open letter to customers
"We have now done tests on 40 of our meat products, no horsemeat was found in any of these tests.
"We did, however, discover that in just two batches of our essential Waitrose frozen British beef meatballs (480g), some of the meatballs may contain some pork.
"In fact, one of the tests carried out showed that the meat in the meatballs was, as it should be, 100% beef.
"But because another test indicated there may be some pork, I felt it important for you to be aware.
"At Waitrose, the welfare of animals and quality of our fresh and frozen meat has always been of the highest priority.
"That is why we have just announced that in partnership with our dedicated beef supplier, we will create a new facility which in addition to supplying all of our fresh beef, will also now produce a range of frozen Waitrose beef products.
"All the beef processed at this site will continue to be sourced from our known and trusted group of British farmers."
Mr Price said the meatballs are safe to eat but customers could return them to stores to be replaced. The affected batches are dated best before end June 2013 and best before end August 2013.
The supermarket chain is to open a new site to make its own range of beef products.