In the latest in a string of food scares in Britain, the Food Standards Agency said that "criminal activity" was likely to blame and ordered further tests on the meat for a veterinary drug.
Findus tested 18 of its beef lasagne products manufactured by supplier Comigel in France and found 11 meals containing 60 percent to 100 percent horse meat, the agency said.
"Findus withdrew the beef lasagne products after its French supplier, Comigel, raised concerns about the type of meat used in the lasagne," the agency said in a statement.
The agency said tests on the lasagne were ordered "as part of its ongoing investigation into mislabelled meat."
"We have no evidence to suggest that this is a food safety risk."
But it said it had ordered further tests on the suspect lasagne for the veterinary drug phenylbutazone, as animals treated with it are not allowed to enter the food chain in Britain.
The agency's chief executive Catherine Brown said it was an "appalling" situation".
"I have to say that that the two cases of gross contamination that we see here indicates that it is highly likely there has been criminal and fraudulent activity involved," she told BBC news.
She added: "We are demanding that food businesses conduct authenticity tests on all beef products, such as beef burgers, meatballs and lasagne, and provide the results to the FSA.
"The tests will be for the presence of significant levels of horse meat."
Findus UK apologised to customers.
"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," a spokesman said.
The spokesman added that "fully compliant beef lasagne will be in stores again soon."
It is the latest horsemeat-related scare after horse DNA was found two weeks ago in beefburgers in Britain and Ireland, countries where horse meat consumption is generally taboo.
Two weeks ago, the Food Safety Authority of Ireland (FSAI) had revealed that up to 29 percent of the meat content of some beefburgers was in fact horse, while they also found pig DNA.
The consumption of horse meat is more common in parts of Europe including France and in central Asia, China and Latin America.