Horse meat has been found in beef products at Taco Bell fast food outlets in the UK.
The Food Standards Agency said it had conducted 1,797 tests over the last seven days.
Over 99% of the tests had come back negative for horse meat levels at or above 1%.
However, it said four tests were positive.
These included Birds Eye ready meals and spicy minced beef skewers from catering supplier Brakes, already named and withdrawn from sale.
It added that no tests to date on samples containing horse DNA have found the veterinary medicine phenylbutazone, also known as bute.
Animals treated with that drug are not allowed to enter the food chain as it may pose a risk to human health.
Tex-Mex fast-food chain Taco Bell, owned by US 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 horse meat," 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 this week 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, 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.
Meanwhile. Beef products sold by Birds Eye, which were pulled from Irish shelves last week, have tested positive for horse DNA.
As a precautionary measure, Birds Eye withdrew all beef products produced by Frigilunch N.V from UK and Irish shelves, namely its Traditional Spaghetti Bolognese, Shepherd's Pie and Beef Lasagne.