The Food Standards Agency has said it will continue testing products "until there's nothing left to find" after the veterinary painkilling drug phenylbutazone - or bute - was found in Asda Smart Price Corned Beef.
The agency's director of operations Andrew Rhodes said customers should not eat the corned beef but added anyone who does is very unlikely to fall ill.
He advised customers to return the product to the supermarket to receive a full refund.
Chief Medical Officer Professor Dame Sally Davies has previously said that 'bute' poses only a very low risk to humans. She said:
Horse meat containing phenylbutazone presents a very low risk to human health.
'Phenylbutazone, known as bute, is a commonly used medicine in horses. It is also prescribed to some patients who are suffering from a severe form of arthritis.
The levels of bute that have previously been found in horse carcasses mean that a person would have to eat 500 - 600 one hundred per cent horsemeat burgers a day to get close to consuming a human's daily dose.
And it passes through the system fairly quickly, so it is unlikely to build up in our bodies.
– Professor Dame Sally Davies, chief medical officer