Tesco's shares fell Wednesday after horse meat was discovered in the supermarket chain's beefburgers.
Tests on beef products sold in British grocers including Tesco, Lidl, Aldi, Iceland and Dunnes Stores, showed low levels of the animal's DNA.
The Food Safety Authority of Ireland (FSAI) have carried out an investigation with Irish authorities which found Britain was one of the areas affected.
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The products were produced by processing plants in Ireland, namely Liffey Meats and Silvercrest Foods, and one in the UK, namely Dalepak Hambleton in North Yorkshire.
Products tested in Ireland contained 37% horse meat, while 85% contained pig DNA. Horse and pig meat were found in 27 beef burger products.
Tim Smith, the Group Technical Director of Tesco, was quoted in London's Daily Telegraph as saying: "The presence of illegal meat in our products is extremely serious. Our customers have the right to expect that food they buy is produced to the highest standards."
Prof Alan Reilly, the Chief Executive of the FSAI, said the reason pig meat was found in products could be due to the fact that different animals are processed in the same plant.
However, he also said there was no clear explanation for the presence of horse DNA in products.
An FSA spokesman said there were continuing to investigate the issue.
Tesco's shares dropped 1.10% to 345.75p at 8:58 Wednesday.
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