Traces of horse meat have been found in burgers on sale in some of the UK and Ireland's busiest supermarkets, the Food Safety Authority of Ireland (FSAI) said.
Scientific tests on beef products sold in Tesco, Lidl, Aldi, Iceland and Dunnes Stores uncovered low levels of the animal's DNA.
A total of 27 products were tested, with ten of them containing horse DNA and 23 pig DNA. Horsemeat accounted for approximately 29% of the meat content in one sample from Tesco.
Professor Alan Reilly, chief executive of the FSAI, said there was no health risk but also no reasonable explanation for horse meat to be found. He said: "The products we have identified as containing horse DNA and/or pig DNA do not pose any food safety risk and consumers should not be worried."
"Whilst there is a plausible explanation for the presence of pig DNA in these products due to the fact that meat from different animals is processed in the same meat plants, there is no clear explanation at this time for the presence of horse DNA in products emanating from meat plants that do not use horse meat in their production process. In Ireland, it is not in our culture to eat horse meat and therefore, we do not expect to find it in a burger."
The retailers have told food safety chiefs they are removing all implicated products from their shelves.
Prof Reilly said traces of other meats would be unacceptable for people who may not eat certain food on religious grounds. He added: "Likewise, for some religious groups or people who abstain from eating pig meat, the presence of traces of pig DNA is unacceptable."
Beef burger products which tested positive for horse DNA were produced by Liffey Meats and Silvercrest Foods in Ireland and one UK plant, Dalepak Hambleton. Silvercrest said it was pulling products from sale and replacing them with new lines.
Some 31 beef meal products such as cottage pie, beef curry pie and lasagne were tested, with 21 found to be positive for pig DNA. All tested negative for horse meat.
The DNA tests found horse in the following products: Tesco Everyday Value Beef Burgers 29.1%; Tesco Beef Quarter Pounders 0.1%; Oakhurst Beef Burgers in Aldi 0.3%; Moordale Quarter Pounders in Lidl 0.1%; Flamehouse Chargrilled Quarter Pounders in Dunnes Stores 0.1%; two varieties of Iceland Quarter Pounders 0.1%. Even lower levels were recorded in Moordale Beef Burgers in Lidl and St Bernard Beef Burgers in Dunnes Stores.