Britain's food regulator said horsemeat had been found in beef products at Taco Bell fast food outlets, the first time it has been discovered on sale in British restaurants since the scandal broke in January.
The Food Standards Agency (FSA) said on Friday it had conducted 1,797 tests over the last seven days, over 99 percent of which had come back negative for horsemeat levels at or above 1 percent.
However, four tests were positive, it said. These included Birds Eye ready meals and Brakes skewers, already named and withdrawn from sale.
It added that no tests to date on samples containing horse DNA have found the veterinary medicine phenylbutazone (bute).
Tex-Mex fast-food chain Taco Bell, owned by U.S. firm Yum Brands Inc, has three outlets in the UK.
"Some batches of ground beef supplied to us from one supplier in Europe tested positive for horsemeat," said Taco Bell UK in a statement.
"We immediately withdrew ground beef from sale in our restaurants, discontinued purchase of that meat, and contacted the Food Standards Agency with this information."
The news is awkward timing for Yum, which on Monday said it was moving to tighten food safety and reverse a sharp drop in business at its KFC restaurants in China after a scare over contaminated chicken.
Earlier on Friday, industry body the British Retail Consortium (BRC) said the latest round of testing by grocers including all the major supermarkets had produced no new positive results.