Beef meatballs withdrawn by Waitrose as they may contain pork were made at a factory earmarked for closure in Scotland, the supermarket said.
The "Essential frozen British beef meatballs" were produced at the ABP Foods-owned Freshlink factory in the east end of Glasgow last summer.
Waitrose said it had checked 40 meat products in light of the horse meat scandal and tests on the meatballs had produced "contradictory results".
Waitrose meatballs are believed to contain pork
The supermarket has withdrawn the 480g range of the product from shelves as a precaution.
In an open letter to customers, Waitrose managing director Mark Price said: "We have now done tests on 40 of our meat products, 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.
"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."
A spokeswoman for ABP 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 FSA (Food Standards Agency) as part of their investigations."
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.
Waitrose has withdrawn the meatballs as a precaution
Price said: "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."
It was announced in November that the Freshlink factory was to close, with production moved to a site in Yorkshire by April.