Tests on Aldi's beef lasagne and spaghetti bolognese have revealed some were entirely made of horsemeat.
The supermarket chain said it has immediately withdrawn its Today's Special Frozen Beef Lasagne and Today's Special Frozen Spaghetti Bolognese from stores as a precautionary measure.
Tests on samples selected at random showed that the products contained between 30% and 100% horsemeat.
The company said it felt "angry and let down" by its French supplier Comigel, which also produced the contaminated Findus beef lasagnes.
Consumers who have purchased the ready meals have been advised to return them to their nearest Aldi store for a full refund.
An Aldi spokesman said: "This is completely unacceptable and like other affected companies, we feel angry and let down by our supplier.
"If the label says beef, our customers expect it to be beef. Suppliers are absolutely clear that they are required to meet our stringent specifications and that we do not tolerate any failure to do so.
"We have acted quickly to withdraw the affected products from sale, conduct additional testing and review the performance of our suppliers.
"The products from Comigel will not be sold in our stores again and we will no longer take any product from Comigel.
"Aldi will not tolerate any supplier who fails to comply with our requirements and we will support any appropriate legal action taken against any supplier that knowingly does wrong."
Aldi said while it was "confident that these products are safe", the products were being tested for the veterinary drug phenylbutazone, or "bute", which is not allowed to enter the food chain in Britain as it may pose a risk to human health.
The revelation came as police joined in the investigation into the deepening horsemeat scandal, as Environment Secretary Owen Paterson told Sky News a "criminal conspiracy" may to be blame.
Officers from Scotland Yard have met with the FSA to discuss its probe.
Mr Paterson, who is holding a summit with food retailers and suppliers today, said it was "shocking" and "totally unacceptable" that products marked as processed beef were found to have contained significant amounts of horsemeat.
"To reassure the public, we are conducting an unprecedented analysis of processed beef products to see how far this either incompetence or negligence, or the criminal conspiracy extends, and we hope to have results by the end of next week," he said.
"Our eyes are very much open to both possibilities, and the FSA have already started working with the Metropolitan Police, who are also working with other police forces across Europe.
"If there is an international element to this, and there is a conspiracy across Europe, we need to work very closely with all our partners ... I am determined that we will work with every possible agency and get to the bottom of this," he added.
Earlier, the Government refused to rule out the possibility that horsemeat might be found in food served in schools or hospitals.
A Number 10 spokeswoman could not say whether any tests had been done on school dinners, hospital and prison meals or other state-provided food.
The French government has told Sky News it has launched a fraud investigation into the horsemeat scandal.
Swedish food giant Findus has withdrawn its frozen beef lasagne products from France and Sweden itself.
The meals were produced in Luxembourg for French supplier Comigel, which in turn had used a Luxembourg supplier called Tavola.
Comigel, the company at the centre of the latest scare, has told the AFP news agency that the meat originated in a Romanian abattoir, and was supplied to Comigel by a meat-processing company called Spanghero, based in southwestern France.