The meat of some beef lasagne products recalled by Findus earlier this week was 100% horsemeat, the Food Standards Agency (FSA) has said.
On Monday Findus withdrew from retailers its beef lasagne in 320g, 360g and 500g sizes as a precaution.
Environment Secretary Owen Paterson said the findings were "completely unacceptable", but Findus said it did not believe it was a food safety issue.
The FSA said companies would now be required to test their beef products.
"In order to get to the bottom of this, we're going to be requiring every company to test every product line," Catherine Brown, the FSA's chief executive, told the BBC.
"If we find any other cases, we will pursue our investigations vigorously until we find out what's happened and put a stop to it."
Ms Brown said it was "highly likely" that criminal activity was to blame for horsemeat being found in some meals.
'Cannot be tolerated'
The FSA said Findus had tested 18 of its beef lasagne products and found 11 meals containing between 60% and 100% horsemeat.
People have been warned not to eat the products, which were made for Findus by French food supplier Comigel.
The FSA said: "We have no evidence to suggest that this is a food safety risk. However, the FSA has ordered Findus to test the lasagne for the veterinary drug phenylbutazone, or 'bute'.
"Animals treated with phenylbutazone are not allowed to enter the food chain as [the drug] may pose a risk to human health.
"The Findus beef lasagne was distributed to the main UK supermarkets and smaller convenience stores. Findus has already begun a full recall of these products.
"People who have bought any Findus beef lasagne products are advised not to eat them and return them to the shop they bought them from."
Mr Paterson said the presence of unauthorised ingredients "cannot be tolerated".
"The responsibility for the safety and authenticity of food lies with those who produce it, and who sell or provide it to the final consumer. I know that food producers, retailers and caterers are as concerned as we are at the course of recent events," he said.
He said the government was working closely with businesses to "root out any illegal activity" and enforce regulations.
"Consumers can be confident that we will take whatever action we consider necessary if we discover evidence of criminality or negligence," he added.
Shadow environment secretary Mary Creagh said the latest revelations raised questions about the extent of the scandal.
"This is no longer just a food safety issue but possibly a criminal trade," she said.
Findus said the product was manufactured by a third party supplier and not by Findus. The frozen food company said all its other products had been tested and were not affected.
Findus said in a statement: "We understand this 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.
"We would like to take this opportunity to apologise to our customers for any inconvenience caused."
This week supermarket chains Aldi and Tesco, as well as Findus UK, withdrew some beef products from sale after concerns were raised at their French supplier.
Comigel alerted Findus and Aldi that their products "do not conform to specification".
They advised them to remove Findus Beef Lasagne and Aldi's Today's Special Frozen Beef Lasagne and Today's Special Frozen Spaghetti Bolognese.
Tesco also decided to withdraw Everyday Value Spaghetti Bolognese.
The Tesco product was produced at the same Comigel site but there was no evidence of contamination, the supermarket said.
The wider food contamination controversy arose in mid-January when Irish food inspectors announced they had found horsemeat in some burgers stocked by a number of UK supermarket chains including Tesco, Iceland and Lidl.
Asda has withdrawn products supplied by Newry-based Freeza Meats which was storing meat found to contain a high proportion of horse DNA. Two samples were found to contain 80% horsemeat.
The horsemeat controversy has hit the Irish meat-processing industry, with a number of suppliers on both sides of the border affected.
The FSA has said it will co-ordinate a UK-wide survey of beef products to test for the possible presence of horse or pig DNA.
There will be "additional emphasis on brands at the lower end of the market, particularly for burger-type products", it said.
Twenty-eight local authorities across the UK will take a total of 224 samples, and results are due to be published in April.
The survey "aims to identify and understand factors that may lead to the presence of meat species that are not labelled as an ingredient, so that this can be explained, eliminated or correctly labelled".