The letter, signed by Philip Clarke of Tesco, Andy Clarke of Asda Stores, Justin King of J Sainsbury and Dalton Philips of Wm Morrison Supermarkets, was published Friday in response to Prime Minister David Cameron's criticism of supermarkets for keeping consumers in the dark about a horse meat scandal crisscrossing Europe, The Daily Telegraph reported.
"We can't accept a situation where the trust customers place in us is being compromised by fraudulent activity or even, as alleged, an international criminal conspiracy," the letter said.
"That is why we are acting together with the Government and the Food Standards Agency, not only to get to the bottom of how this has happened but to take whatever steps are necessary to reassure customers that they can trust the food they buy.
"We are working around the clock to complete the most comprehensive testing of processed beef products ever undertaken, anywhere in the world."
Cameron said Thursday retailers should have explained to the media how horse meat ended up on their shelves.
"It is not acceptable for retailers to remain silent while their customers have been misled. The supermarkets need to justify their action and reassure the public," he said.
The Lancashire County Council said tests by the Food Standards Agency determined cottage pies sent to 47 schools in the county contained traces of horse DNA. This is the first report of the tainted beef finding its way into school cafeterias, the Telegraph reported.
"We share the concerns people have about what is clearly a major problem in food supplies across the U.K. and Europe," said council member Susie Charles.
"Because of those concerns we decided to seek extra assurance that our external suppliers were not providing any products containing horsemeat DNA, and one of the products has returned a positive result," she said.