In the case of one of the Tesco products, horse DNA was found in almost 30pc of the meat content in the tests.
A number of investigations were ongoing today.
Tesco's Technical Director Tim Smith said: "We know that there are two batches implicated in the survey done by the authorities and we don't know how many of those products have been sold.
"Our investigation of the supplier will cover in great detail exactly what happened, who was responsible for it and exactly how long this has been going on. It's critical that we get to the bottom of those facts so that it never happens again."
Tesco said that the firm now believes that the presence of horse meat in some of its products was caused by "illegality" or "gross negligence".
"The important thing to say is there is only two ways this could happen -- one of them involves illegality by suppliers or suppliers to those individual suppliers or gross negligence in which case we are very keen to get to the bottom of this," Mr Smith added.
Nine of the 10 samples were found to have contained low traces of horse DNA, less than 0.1pc in some cases. However the level of DNA in a Tesco Everyday Value Beef Burger was as high as 29.1pc.
High level probes are focussing on two Irish meat processing plants, based in Cavan and Monaghan, as well as a facility in the UK.
One of the plants, Silvercrest, today said it believed the problem was caused by a "raw material" shipped in from abroad.
The disturbing report from the Food Safety Authority of Ireland (FSAI) was also dominating media in Britain today.
It revealed that Tesco, Lidl, Aldi, Dunnes Stores and Iceland have all sold burgers containing horse and pig meat.
All of the retailers have withdrawn the products affected as health and food bodies issued assurances that there is no risk to public health.
The FSAI sampled a total of 27 burger products and found that 10 of these were contaminated. Traces of pig meat was also found in 23 of the items.
The FSAI emphasised that there were no public health concerns, but admitted that it may carry out probes in other plants.
The revelations have rocked the multi-billion euro Irish food industry and raised fears that the scandal could affect exports.
The matter was brought to the attention of the Cabinet yesterday as Agriculture Minister Simon Coveney vowed to "get to the bottom of it".
One of the plants at the centre of the scandal, Silvercrest in Co Monaghan, said that a product from non-Irish suppliers is believed to have contained traces of horse meat.
"Products that we bought on the understanding that they were beef based products would appear to have traces of equine. What we understand we were buying was not what we actually received," a spokesman said.
The other plant affected, Liffey Meats, said it was also investigating "imported ingredients".
"The company believes it has identified the source of the contamination. Liffey Meats is purely a beef processor and has absolute traceability on all of the beef used. The source of the contamination is imported ingredients and these will be replaced from other sources before production is resumed and customers are supplied."
Chief Executive of Bord Bia, Aidan Cotter, said that the scandal has come as a "surprise" to those in the beef industry.
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