The Co-operative Group and Asda have said several of the beef burgers they withdrew from sale as a precaution amid the horse meat scandal featured equine DNA and The Co-op has delisted supplier Silvercrest.

In tests commissioned by The Co-op, four samples of 17 tested contained equine DNA. Three samples had less than 1% traces and in one sample the percentage was 17.7%.

The sample is the second largest trace found behind a Tesco sample which featured 29.1% horse DNA. Tesco and The Co-op have both delisted supplier Silvercrest.

The results are a blow for The Co-op which had withdrawn the burgers as a precaution on January 16 and markets its transparent, ethical supply chain to shoppers.

The retailer said it will tighten its “already stringent quality checks”.

Asda said it had found “traces” of horse DNA in four of 29 products it withdrew from sale.

A spokeswoman said: “We’ve not found any horse meat. But four products have tested positive for very small trace elements of horse DNA.”

Food Standards Agency chief executive Catherine Brown today told MPs that the unnamed Polish supplier responsible for the contimination has been selling beef “filler” to an Irish meat processor for over a year.

A Co-operative spokeswoman said: “We pride ourselves on the quality of the products we sell, and we are taking this matter very seriously. Our decision to withdraw these products at the first opportunity and cease taking further product from this site, has proven to be the correct course of action.

“Whilst there are no safety issues involved, it is now apparent that some of the withdrawn products have not met the high standards we and our customers expect. We apologise for this.”

She added: “We specify that all meat in our frozen burgers should be 100% British, but we now strongly believe that some of the meat used to produce these burgers came from outside the UK and was not British in origin, and as a result we have taken the decision to delist Silvercrest as a supplier with immediate effect.”