Tests on four beef products sold by Birds Eye, Taco Bell and catering supplier Brakes have been found to contain horsemeat.
Checks revealed contamination of Birds Eye Traditional Spaghetti Bolognese and Beef Lasagne, Taco Bell's ground beef and Brakes' spicy minced beef skewer, the Food Standards Agency said.
Ten tests on the four products returned results of more than 1% horsemeat, the FSA said, and all four have been withdrawn from sale.
US-owned Tex-Mex restaurant chain Taco Bell said that it was "disappointed" to have discovered the horsemeat in tests it carried out on beef supplied to its UK restaurants by a sole European supplier.
"We immediately withdrew ground beef from sale in our restaurants, discontinued purchase of that meat, and contacted the Food Standards Agency with this information," it said in a statement.
"We would like to apologise to all of our customers, and we can reassure you that we are working hard to ensure that every precaution is being undertaken to guarantee that we are only supplied with products that meet the high standards we demand."
Overall, the Food Standards Agency has received 5,430 test results
Birds Eye had already withdrawn the spaghetti bolognese, lasagne and a third ready meal, a shepherd's pie, from sale in Britain and the Republic of Ireland as a precaution after tests found 2% of horse DNA in a chilli con carne dish it sold in Belgium.
They are made by the same Belgian manufacturer, Frigilunch NV.
"No other Birds Eye products have tested positive for horse DNA, nor do they share the same supply chains as Frigilunch NV," the company said in a statement.
"Going forward we are introducing a new ongoing DNA testing programme that will ensure no minced beef meat product can leave our facilities without first having been cleared by DNA testing."
Brakes, which is based in Ashford, Kent, is the supplier for the House of Commons Catering Service and last month it withdrew its steak and kidney pie, beef and onion pie, steak and kidney suet pudding, and beef Italian meatballs as a precaution.
It also supplies pubs among 19,000 customers who buy around 48,000 cases of products containing beef every week.
It too said it was introducing new tests after the discovery, alongside 259 negative tests.
"Our tests also confirmed one positive equine DNA finding at between 1% and 10% on a Brakes spicy minced beef skewer and one positive test reported by a customer of our subsidiary division Creative Foods, on a lasagne manufactured exclusively for them," it said.
"Brakes have also segregated a frozen burger as a precaution after equine DNA at 1% was reported to the Food Standards Agency.
"Brakes and Creative Foods are very disappointed to have been let down by our respective suppliers and have sincerely apologised to our customers.
The latest findings come in the third round of checks on products
"As any responsible company, we have a duty of care to all our customers and the consumers they serve to guarantee the integrity of the products we purchase."
The discoveries were made in the third round of tests carried out since January.
A total of 19 products have now been confirmed to contain over 1% of horse DNA.
No tests to date on samples containing horse DNA have found the veterinary drug phenylbutazone, or bute, to be present.
Across the industry, a first wave of tests found horse meat in products including Aldi's special frozen beef lasagne and special frozen spaghetti bolognese, Co-op frozen quarter-pounder burgers, Findus beef lasagne, Rangeland's catering burger products, and Tesco Value frozen burgers and Value spaghetti bolognese.
A second wave of tests revealed contamination of Asda's chilled beef bolognese sauce, beef burgers, minced beef and halal minced beef sold by Sodexo, which supplies food to schools, care homes and the armed forces, and a Whitbread Group lasagne and beef burger.
Earlier, the British Retail Consortium said test results for horsemeat in all minced beef lines used by the UK's largest supermarkets had revealed no new cases of contamination.
Some 95% of products sold by retailers have now been checked, it said, with the latest round of checks since February 22 including 361 tests on 103 products.
A total of 1,889 tests have been carried out by the trade organisation's members since January 20, with 0.3% of them finding contamination.
Meanwhile in Germany, authorities say they have found a carcinogenic substance in animal feed delivered to more than 3,500 farms - but stressed that any risk to humans was unlikely.
Aflatoxin B1 is a chemical produced by fungus that can grow on hay or grains and appear in the milk of animals that eat the mildewed feed.
The state agriculture ministry in Lower Saxony said the contamination originated from a shipment of corn from Serbia.
It said it did not believe there was any danger to consumers and there was no indication legal limits on aflatoxins in milk had been exceeded.