Tesco has dropped a major supplier after finding horsemeat in a range of its spaghetti bolognese ready meals.
The supermarket giant found levels of horse DNA exceeded 60% in tests on its Everyday Value Spaghetti Bolognese, which were taken off shelves last week as a precaution.
Frozen food firm Findus and Aldi found the meat in products made by the same company, Comigel, and Tesco joined them in dropping the French company as a supplier.
The announcement came as Britain's shadow environment secretary Mary Creagh claimed tens of thousands of horses have disappeared in Northen Ireland and the Government demanded the immediate testing of all suspected products.
The retailer said that of the positive results on the spaghetti meals, most were at a trace level - but three showed significant levels of horse DNA, exceeding 60%.
Tesco group technical director Tim Smith said Everyday Value Spaghetti Bolognese was meant to contain only Irish beef.
"The source of the horsemeat is still under investigation by the relevant authorities," he said.
"The level of contamination suggests that Comigel was not following the appropriate production process for our Tesco product and we will not take food from their facility again.
"We are very sorry that we have let customers down. We set ourselves high standards for the food we sell and we have had two cases in recent weeks where we have not met those standards.
"Our DNA testing programme is under way and will give us and our customers assurance that the product they buy is what it should be."
Last month, Irish food inspectors said they had found horsemeat in some burgers stocked by a number of UK supermarket chains, including Tesco, Iceland and Lidl.
And last week frozen foods firm Findus announced it had taken its beef lasagnes made by Comigel off shelves after some were found to have up to 100% horsemeat in them.
The scandal has spread all over the continent as details of the elaborate supply chain in the meat industry emerge.
Environment Secretary Owen Paterson told the House of Commons that it appeared that "criminal activity" had been at the heart of the scandal.
He told MPs there would be immediate testing of products across the supply chain, including tests at schools, hospitals and prisons.
The Food Standards Agency had also reassured him that the products recalled did not present a risk to the public, but consumers who had bought the Findus beef lasagnes should return them to the shop they had bought them from as a "precaution".
He said the "ultimate source" of the problem was not yet known but agencies were investigating a supply network that stretched across Europe.
Mr Paterson said: "At the moment this appears to be an issue of fraud and mis-labelling, but if anything suggests the need for changes to surveillance and enforcement in the UK we will not hesitate to make those changes."
Shadow environment secretary Mary Creagh claimed there were 70,000 horses unaccounted for in Northern Ireland, with unwanted animals given false paperwork before being sold for 10 euros (£8) and then resold to dealers for meat for as much as 500 euro (£423).
Speaking in the Commons, Ms Creagh said: "The Ulster Society for the Prevention of Cruelty to Animals have clear evidence of an illegal trade of unfit horses from Ireland to the UK for meat, with horses being re-passported to meet demands for horsemeat in mainland Europe.
"It is very convenient to blame the Poles and the Romanians but so far neither country have found any problems with their beef abattoirs."
Following earlier cases of contamination, supermarkets across Europe accused Romanian suppliers of being the source of the contaminated meat.
But the country's ambassador to the UK, Dr Ion Jinga, told Sky News that they were not to blame, saying Romanians love horses as much as people in Britain do.
Earlier on Monday, Romanian Prime Minister Victor Ponta insisted any fraud committed in the scandal did not happen in his country.
"Romania cannot accept to be the usual suspect," he told reporters.
"I am very angry, to be very honest."