The Co-operative Group CEO Peter Marks has emailed more than a million of its members to apologise for the horsemeat scandal.

The society withdrew two own-label frozen burger products as a precaution last month, but they were later found to contain 17.7% horse DNA.

“I believe that as a result of this food scandal we have let you down,” Marks wrote in the email.

“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 gave an update on the society’s testing for horse DNA, and said that the first 76 of 102 products had proved negative. Further results are expected at the end of the week.

On the lines that were withdrawn, Marks wrote: “Previously, we removed two lines of frozen own brand beef burgers which were found to contain horsemeat DNA which had been supplied by Silvercrest Foods, Co Monaghan, Ireland. We specify that all meat in our frozen burgers should be 100% British, and as a result we have taken the decision to delist Silvercrest as a supplier. In addition, we are tightening our quality checks to ensure our products meet the high specifications that we set on behalf of our customers.”

He added that the society would review all of its meat supply chain and “put in place an enhanced process with greater robustness and transparency to ensure that we know the full provenance of all of our meat products”.

“In fact, our testing across all of our food products is to be stepped up. To achieve this we will have to carry out more stringent checks, However, I can assure you that any additional cost will not be passed onto you. We do not expect our customers to pay for this necessary rigour,” Marks wrote.