Criminal activity is "highly likely" to be to blame after some Findus beef lasagnes were found to contain 100% horsemeat.
Consumers have been warned not to eat the meals and the Food Standards Agency (FSA) has ordered all food companies to test their beef products and provide the results by February 15.
Findus has said it was told on Saturday that its French supplier, Comigel, could not guarantee what products were in the lasagnes.
But a letter obtained by Labour MP Tom Watson says the foods may have been contaminated since last summer.
The letter from Findus to retailers warns that Comigel told it that raw materials delivered to it since August 1 last year were "likely to be non-conform and consequently the labelling on finished products is incorrect".
Findus analysed 18 of its beef lasagne meals and found 11 contained between 60% and 100% horsemeat, the FSA said.
Its 320g, 360g and 500g meals were withdrawn from supermarket shelves on Monday.
Prime Minister David Cameron said it was a "very shocking story, it's completely unacceptable", and conceded there was "great public concern" about the issue.
"People will be angry to find out they have been eating horse when they thought they were eating beef.
"It's important we get this right. It's important to say there's no reason to believe any frozen food currently on sale is unsafe or a danger to health.
"But it's not about food safety - it's about proper food labelling and about confidence in retailers. The Food Standards Agency's got to do everything it can, retailers have got to do everything they can to make sure the food they sell is accurately labelled and described."
Downing Street described the latest incident as "distasteful" but stressed there was no evidence of a health risk and urged consumers to follow the FSA's advice.
A Number 10 spokeswoman said the matter appeared to involve "acts of criminality" which were being investigated by the police, including officers from the UK.
She could not say, however, whether any tests had been done on school dinners, hospital and prison meals or other state-provided food.
The French government has said it is investigating whether any fraud has been committed over the mis-selling and labelling of meat products.
The FSA has confirmed tests have been ordered on the lasagne for the veterinary drug phenylbutazone or "bute", which is banned from entering the food chain in the UK.
Findus has apologised for letting customers down and said refunds would be offered to anyone who bought the affected products.
A spokesman said: "We understand this it is a very sensitive subject for consumers and we would like to reassure you we have reacted immediately. We do not believe this to be a food safety issue.
"We are confident that we have fully resolved this supply chain issue. Fully compliant beef lasagne will be in stores again soon.
"We would like to take this opportunity to apologise to our customers for any inconvenience caused."
Comigel advised them to remove Findus beef lasagne and Aldi's Today's Special frozen beef lasagne and Today's Special frozen spaghetti bolognese from shelves.
Shadow environment Secretary Mary Creagh warned people to be careful about what they are eating.
"I also think we need to get into the schools, hospitals, catering establishments, find out what's in the freezers and test those as well. We have to have confidence that what we are eating is safe," she told Sky News.
Tesco has also decided to withdraw its Everyday Value spaghetti bolognese, which is produced at the same Comigel site.
A spokesman said: "We are aware of the results of the Findus tests and we will of course assist Findus with their recall process.
"Tests on our frozen Everyday Value spaghetti bolognese product are ongoing under our new DNA testing programme. We will inform our customers of the results as soon as possible."
The FSA, Defra and the Department of Health are working with businesses and trade bodies to enforce food safety and assess whether there are significant levels of improperly described meat in a whole series of processed beef products in the UK, including supplies to schools and hospitals.
Environment Secretary Owen Paterson said: "The presence of unauthorised ingredients cannot be tolerated ... the responsibility and for the safety and authenticity of food lies with those who produce it, and who sell or provide it to the final consumer."
Anyone who has purchased a Findus beef lasagne can call the firm's UK customer care line on 0800 132584, those in the Republic of Ireland, 1800 800500, or email firstname.lastname@example.org for a full refund.