British supermarkets are selling beefburgers containing horse meat, according to a British news website.
The Mail Online reports that tests on products from four chains show a third were contaminated. It said most had only small traces but in one product from Tesco 29 per cent of the 'beef' content was found to be horse meat.
"Last night, Tesco issued an apology and said the suspect burgers had been withdrawn. It is likely however that thousands have been eaten, with many more still stored in home freezers," the website said.
"The highest level of horse meat was found in the chain's Everyday Value beefburgers but traces were also detected in its frozen quarter pounders.
"The revelations emerged following checks by Irish food safety experts."
The affected burgers were made at two sites in Ireland and one in North Yorkshire owned by Dalepak Foods.
Other products that tested positive for horse DNA included Aldi's Oakhurst Beef Burgers and Lidl's Moordale Beef Burgers.
The retailers have told food safety chiefs they are removing all implicated products from their shelves.
Aldi said its product was on sale only in Ireland and had been withdrawn.
Traces were also detected in batches of raw ingredients, including some imported from The Netherlands and Spain.
Tim Smith, Tesco's technical director and a former head of the Food Standards Agency, said the company understood that many customers would be disturbed by the news.
It is not illegal to buy or sell horse meat in the UK, but it is illegal not to declare every ingredient on food labels.