The horsemeat scandal continues to affect Britain‘s top grocers after both Tesco and Waitrose were found to be selling potentially contaminated burgers, it has emerged today.

Grocery giant Tesco has apologised today after a BBC reporter was able to purchase its own-brand ‘Free From‘ frozen burgers which, though not directly implicated in the ongoing horsemeat scandal, were said to have beehn removed from stores as a precaution pending further investigation.

A member of Tesco‘s staff overrode a till alert message when the reporter tried to buy the item at a branch in Oxford, the BBC said, and Tesco has apologised for the error.

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said: “Whilst this product was not implicated in the Food Safety Authority of Ireland (FSAI) investigation, and was withdrawn as a precaution, we are urgently investigating how this product came to be on a shelf in store.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

Earlier this month, the FSAI announced that it was investigating food standards following the analysis of beef burger products sold in supermarkets which found that 37 per cent of 27 tested products tested positive for horse DNA while 85 per cent were found to contain pig DNA.

Supermarkets Tesco, Aldi, Iceland and Dunnes Stores were each implicated in the scandal and today, upmarket rival Waitrose has been dragged into the spotlight after it emerged that all of its frozen burgers are supplied from one of the sites where the DNA was discovered.

This includes burgers from its Essential Waitrose range, which the grocer announced today had seen sales soar 21 per cent in the week to January 19th 2013.

While the grocer has now removed frozen burgers from its shelves, this news will cause embarrassment for the retailer as its Managing Director told news agency Reuters yesterday that the scandal was unsurprising given ongoing price wars among supermarkets.

“If you have a competition that says: Who can sell the cheapest stuff? Inevitably at a point in time you will get something like this,” Reuters quoted him as saying.

Today however, Waitrose has issued a statement announcing that it is suspending frozen burger sales as a precautionary measure.

A spokesperson commented: “All our beef comes from our own British suppliers (we only use beef from British farms in all our fresh and frozen burgers).

“As a consequence we are 100 per cent confident in the integrity of our supply chain.

“The ingredients in our burgers are simple with all meat traceable back to British farms that we know.

“Our frozen burgers (which make up 8 per cent of our total burger sales) are produced by Dalepak.

“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).

“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. (We understand the accreditation was reinstated on Thursday 24.1.13).

“As a further precaution our frozen burgers were sent for testing and they tested negative for any species other than beef. “