Batches of Waitrose beef meatballs that were withdrawn from shelves for containing pork were made at a factory in Glasgow, it has emerged.
The supermarket chain has confirmed that the items were made by ABP Freshlink in Old Shettleston Road.
ABP Food Group responded: "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."
Republic of Ireland-based ABP said its Glasgow convenience foods factory, which is due to close in April, would share its own results with the Food Standards Agency as part of its investigations.
Waitrose said that, within three months, it will switch the manufacture of its convenience foods to a new unit at Dovecote Park's Yorkshire facility, which has been exclusively supplying the chain with fresh beef products since 1997.
It announced the discovery of traces of pork after it carried out tests of 40 of its products in the wake of the horse meat controversy.
Safe to eat
Waitrose spokesman Mark Price said in a website statement: "No horse meat 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."
Waitrose stressed that one test had shown the meatballs to be 100% beef but decided to issue a statement to customers after a second test showed some pork.
"Although the meatballs are safe to eat, pork is not listed as an ingredient and should not be part of the recipe," said Mr Price.
"The affected date codes are best before end June 2013 and best before end August 2013. No other date codes or products are affected."
Waitrose has offered a refund to customers should they wish to return the products.
The company, part of the John Lewis Partnership, said it would be creating a new facility to supply its own beef products.
"At Waitrose, the welfare of animals and quality of our fresh and frozen meat has always been of the highest priority," added Mr Price.
"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.
"We only stock fresh and frozen beef, pork and chicken from British farmers - and that includes the meat in our ready meals.
"The only exception is lamb, which is a seasonal meat and requires some New Zealand lamb to make up for our requirements outside the UK season."