The Environment Secretary is to hold another round of talks with the food industry to discuss the horsemeat scandal.
Owen Paterson has told MPs that it appears "criminal activity" is at the heart of the scandal that has spread across Europe.
He will hold talks with the Food Standards Agency (FSA) and food industry representatives tonight in what will be the second UK summit in a matter of days.
Ministers from the countries affected are also due to meet in Brussels on Wednesday to discuss what wider action needs to be taken.
The supermarket found its frozen Everyday Value Spaghetti Bolognese, which was withdrawn last week as a precaution, contained more than 60% horsemeat.
Tim Smith, Tesco's group technical director, said: "The source of the horsemeat is still under investigation by the relevant authorities.
"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."
Findus, whose beef lasagne supplied by Comigel was found to contain up to 100% horsemeat, is considering legal action after an internal investigation indicated the contamination was "not accidental".
Further tests are now being run on products across the supply chain in the UK, including at schools, hospitals and prisons.
The results will be published on Friday, with Mr Paterson warning the public to brace for "more bad news".
He said the "ultimate source" of the problem was not yet known but agencies are investigating a supply network that stretches across Europe.
Mr Paterson said: "At the moment this appears to be an issue of fraud and mislabelling, 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 horses in Northern Ireland were being illegally sold to dealers for meat for hundreds of pounds.
"It is very convenient to blame the Poles and the Romanians but so far neither country has found any problems with their beef abattoirs," she said.
Conservative MP Anne McIntosh, chairwoman of the Environment, Food and Rural Affairs select committee, has backed a ban on meat imports and urged the public to buy their meat locally.
Butchers are already reporting a marked spike in trade as consumers turn their backs on imported and processed goods as the extent of the contamination grows.
Yorkshire butcher Brindon Addy, the chairman of the Q Guild which represents 130 butchers across England, Scotland and Wales, said: "It is obviously great news for those butchers who have found it difficult to compete with the big supermarkets in the past.
"People slip into the convenience of supermarket shopping, but whenever there is a scare - be it horsemeat or BSE - they always come back."
The National Beef Association (NBA) has called for more precise labelling and the addition of words "United Kingdom origin" to help consumers.
An initial French investigation has revealed that the horsemeat ended up in Comigel's Luxembourg factory, supplied by a French firm, and that a Dutch and Cypriot trader had also been involved.
However, the meat originally came from a Romanian abattoir.
Romanian prime minister Victor Ponta insists his government has found no evidence that any companies in Romania had broken any European laws.
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