Peter Marks, chief executive of the Co-op, wrote to more than a million of the company's members today to apologise for "meat contamination" in its products.
Two lines of frozen own-brand beefburgers, made with meat supplied by Irish manufacturer Silvercrest Foods, were withdrawn from sale by the firm after they were found to contain horse DNA.
Mr Marks wrote:
I believe that, as a result of this food scandal, we have let you down. The discovery of meat contamination in two of our own-brand products has caused you to question the trust that you can place in us as a food retailer.
I strongly believe that all food retailers must accept ultimate accountability for the products we sell to our customers. We cannot blame the Government or the regulators, or even our suppliers. At the end of the day, the buck stops here.
He also told members that the first 76 products of 102 own-brand products sent for testing were negative for horse meat contamination, with further results expected later.
The Co-op is to review its meat supply chain in light of the scandal, he added.
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