Tesco has launched an urgent investigation after one of its stores sold burgers to the public that had been withdrawn as a result of the horse DNA scandal.

The burger – an own-label free-from quarter pounder – triggered an alert warning “this product has been withdrawn from sale” at the checkout, but staff at the store in Cowley, Oxfordshire, overrode the warning and sold the product anyway.

The problem was uncovered by reporters at BBC Oxford following a tip-off from a member of the public.

The burger was not among those that had tested positive for horse DNA in a survey by the Food Safety Authority of Ireland, but Tesco had withdrawn it as a precaution and it therefore should not have been on shelves and available for sale. The withdrawal was voluntary, and there is no evidence of any food safety problems with any of the burgers that have been removed from sale.

Tesco said it was urgently investigating how the product came to be sold to the public. A spokesman added: “Whilst this product was not implicated in the FSAI investigation, and was withdrawn as a precaution, we are urgently investigating how this product came to be on a shelf in store. The  block on purchase at the checkout should not have been overridden. We sincerely apologise for this, and we have spoken to the store to ensure that this does not happen again.”

Waitrose removes burgers

The news comes as Waitrose has voluntarily removed its own-label frozen burgers from sale, after its supplier – Dalepak Hambleton, owned by ABP Food Group – temporarily had its BRC accreditation withdrawn. Waitrose stressed the move was a precaution and that it was “100% confident in the integrity of our supply chain”.

“Our technical team visited the Dalepak site last week and were happy that our products were produced to our high specification and separately from other companies’ products – ours are produced at 6am before any other burgers,” a spokeswoman said. “However, when the British Retail Consortium suspended their accreditation of the site, we decided – as a precautionary measure – to take our frozen burgers off sale.  It added that it understood Dalepak’s BRC accreditation was reinstated yesterday (24 January).

In its original statement, Waitrose also said it had sent its frozen burgers for DNA testing “and they tested negative for any species other than beef”. However, a spokeswoman later clarified the burgers had been tested specifically for horse, chicken, pork and lamb, and had tested negative for those.

DNA tests can only be used to verify the presence or absence of the DNA of a specific species; they cannot be used to check generically if a product contains anything other than, say, beef.