Tests on Findus beef lasagne have revealed that some of the ready meals were made entirely from horsemeat.
Findus analysed 18 of its beef lasagne products and found 11 meals contained between 60% and 100% horsemeat, the Food Standards Agency (FSA) said.
There is no evidence to suggest the horsemeat found in the Findus beef lasagne is a food safety risk, the FSA said.
However, the agency has ordered urgent tests on the lasagne for the veterinary drug phenylbutazone. Meat from animals treated with "bute" is not allowed to enter the food chain in Britain as it may pose a risk to human health.
All food companies have been told to test their beef products, with the FSA saying it was "highly likely" that criminal activity was to blame for the contamination.
Consumers who have purchased the ready meals - produced by French food supplier Comigel on behalf of Findus - have been advised by the FSA not to eat them and return them to the shop they were bought from.
Retail giant Tesco and discount chain Aldi have already withdrawn a range of ready meals produced by Comigel over fears they contained contaminated meat.
Findus UK has already started a full recall of its lasagne products. It withdrew its 320g, 360g and 500g lasagne meals from supermarket shelves as a precautionary measure earlier this week.
It came after Comigel alerted Findus and Aldi that their products "do not conform to specification".
It advised them to remove Findus beef lasagne and Aldi's Today's Special frozen beef lasagne and Today's Special frozen spaghetti bolognese.
Findus UK apologised to customers "for any inconvenience caused" - and said anyone who bought the affected lasagne products could get a full refund.
A spokesman said: "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. Fully compliant beef lasagne will be in stores again soon."
Tesco also decided to withdraw its Everyday Value spaghetti bolognese, which is produced at the same Comigel site.
A Tesco 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."
Labour has accused ministers of being "asleep on the job" and has called for a police investigation into what it believes is fraud.
Shadow environment secretary Mary Creagh said she was "shocked and appalled" by the latest revelations.
She told Sky News: "The time has come for government ministers to pull their heads out of the sand and to take some swift action.
"We have had three weeks of damaging revelations about what is happening in the meat industry ... there is evidence that criminal gangs are involved in this, and frankly I cannot believe that the Government hasn't called in the police to investigate this in the UK.
"I don't see how we get to the bottom of it without getting in specialist teams and working out who is behind this fraud and why it is happening."
People must have confidence that the food they buy is properly labelled, legal and safe to eat, she added.
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.