Investigations are being urgently carried out by the meat companies implicated in the investigation by the Food Safety Authority of Ireland (FSAI), which saw one Tesco beef burger discovered to contain 29% horsemeat.
A source close to ABP, which owns two of the businesses named by the FSAI, Silvercrest Foods and Dalepak Hambleton, which supply Tesco, said they were stunned by the discovery.
Describing it as a "rogue" sample, he said it would be the focus of the organisation’s investigation: "We’re shocked by that result and we’re at a loss to explain why that result is the way it is. Our investigations are centred around meat and meat products from two suppliers, and technical auditors have been sent into those companies.
"We’re also carrying out a large number of DNA samples of our own, and we’re hoping to have the results back by Friday."
The source refused to name the suppliers but said they were EU-based businesses, which supplied them with meat and beef ingredient products. He added that ABP would now also be implementing a new regime to test meat product DNA from suppliers, which he said would be "industry-leading".
In a statement, an ABP spokesman later added: "Our group companies only buy meat from licensed and approved EU suppliers. These results relate only to where beef products have been sourced by those suppliers from the Continent. Only a small percentage of meat is currently procured from outside of the UK & Ireland. Fresh meat products are unaffected.
"Should our own testing prove positive, we are also considering our options in respect of the two suppliers concerned. It is vital that the integrity of the supply chain is assured and we are committed to restoring consumer confidence."
Meanwhile, the third company implicated, Liffey Meats, said in a statement: "In our case, equine DNA was detected in three products. However it must be emphasised that the levels of detection were minute.
"Liffey Meats has never produced, purchased or traded any equine products. We do import some raw ingredients as part of our manufacturing process. As part of its analysis, FSAI identified traces of equine DNA in some of those raw ingredients.
"We now believe that such imported raw ingredients were the ultimate source of the DNA traces found in some of our products."