Tesco has apologised and launched an urgent investigation after burgers that had been withdrawn from sale as a result of the horse meat scandal were sold to the public.

Its own-brand Free From burgers were sold at its store in Cowley, Oxfordshire despite an alert warning at the checkout that “this product has been withdrawn from sale”. Although the burger was not found to include horse meat, the supermarket had decided to withdraw the line of quarter pounder burgers as a precaution.

A Tesco spokesman said: “Whilst this product was not implicated in the Food Standard Authority of Ireland investigation, and was withdrawn as a precaution, we are urgently investigating how this product came to be on a shelf in store.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.”

The incident comes as Waitrose has admitted to removing six product lines from its stores after its supplier Dalepak, one of the firms involved in the horsemeat scandal, temporarily lost its BRC food standards accreditation.

A Waitrose spokesman said the burgers were sent for testing and were found to be 100% beef. “As a consequence we are 100% confident in the integrity of our supply chain.”

He added: “When the BRC suspended their accreditation of the site we decided - as a precautionary measure - to take our frozen burgers off sale.” The suspension of the site’s accreditation has since been lifted.

Its managing director Mark Price had previously criticised supermarkets for selling cheap food. “If you have a competition that says, ‘Who can sell the cheapest stuff?’ inevitably you will get something like this.”