LONDON — The U.K. Food Standards Agency said it is working with police in its investigation of mislabeled meat after tests confirmed this week that a sample of Findus frozen beef lasagna contained more than 60% horse meat.

Both the lasagna and supermarket chain Tesco’s frozen beef burgers, which tested positive to contain approximately 29% horse meat, have been pulled from store shelves in the U.K.

“This is a very serious issue,” the agency said in a statement. “The evidence we have about the two cases, of the significant amount of horse meat in burgers and lasagna, points to either gross negligence or deliberate contamination in the food chain. This is why we have already involved the police, both here and in Europe.”

The presence of horse meat is not a food safety hazard unless the horses were treated with veterinary drug phenylbutazone, which may pose a risk to human health, the agency said. The agency ordered Findus to test its product for the drug and has advised consumers to return Findus lasagna and Tesco burgers as a precaution.

“We believe that these two particular cases, the frozen burgers from Tesco and the lasagna from Findus, are linked to suppliers in Ireland and France respectively,” the agency said. “We are working closely with the authorities in these countries to get to the root of the problem. Our priority remains to protect U.K. consumers. People have been asking whether it is safe to eat any frozen meat products at the moment. There is no reason to suspect that there’s any health issue with frozen food in general, and we wouldn’t advise people to stop eating it.”

Millions of burgers have been removed from sale in the past month following an analysis by the F.S.A. of Ireland, which examined the labeling accuracy of beef products. The investigation escalated after the agency found 10 of 27 beef burger samples contained horse DNA and 23 contained pig DNA.

In collaboration with the Department for Environment, Food and Rural Affairs in London, the F.S.A. established a protocol for meat testing.

“Following our investigations into Findus products, the F.S.A. is now requiring a more robust response from the food industry in order to demonstrate that the food it sells and serves is what it says it is on the label,” said Catherine Brown, chief executive of the agency. “We are demanding that food businesses conduct authenticity tests on all beef products, such as beef burgers, meatballs and lasagna, and provide the results to the F.S.A. The tests will be for the presence of significant levels of horse meat.”

The issue of food mislabeling surfaced in January when the U.S. Pharmacopeial Convention, Rockville, Md., added nearly 800 new records to its food fraud database. Olive oil, milk, saffron, honey and coffee were among the ingredients identified to contain a deliberate substitution, addition, tampering or misrepresentation in its ingredients or packaging.