Horse DNA has been found in some beef burgers being sold in UK and Irish supermarkets, the Republic of Ireland's food safety authority (FSAI) has said.
The FSAI said the meat came from two processing plants in Ireland, Liffey Meats and Silvercrest Foods, and the Dalepak Hambleton plant in Yorkshire.
It said they posed no health risk.
The burgers were on sale in Tesco and Iceland in the UK and Ireland. In the Republic of Ireland they were on sale in Dunnes Stores, Lidl, and Aldi.
The FSAI said the retailers have stated that they were removing all implicated batches of the burgers.
A total of 27 products were analysed, with 10 of them containing horse DNA and 23 containing pig DNA.
Horsemeat accounted for approximately 29% of the meat content in one sample from Tesco.
In addition, 31 beef meal products, including cottage pie, beef curry pie and lasagne, were analysed, of which 21 tested positive for pig DNA.
The chief executive of the FSAI, Professor Alan Reilly, said that while the findings posed no risk to public health, they did raise some concerns.
"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 horsemeat in their production process," he said.
"In Ireland, it is not in our culture to eat horsemeat and therefore, we do not expect to find it in a burger," Professor Reilly added.
"Likewise, for some religious groups or people who abstain from eating pig meat, the presence of traces of pig DNA is unacceptable."
Tesco's group technical director, Tim Smith, said his company was informed of the test results by the FSAI on Tuesday and they "immediately withdrew from sale all products from the supplier in question".
In Tesco's case, two frozen beef burger products that are sold in both the UK and Ireland were found to contain horse DNA.
In a statement, Mr Smith said: "The safety and quality of our food is of the highest importance to Tesco. We will not tolerate any compromise in the quality of the food we sell. The presence of illegal meat in our products is extremely serious."
He added that Tesco was "working with the authorities in Ireland and the UK, and with the supplier concerned, to urgently understand how this has happened and how to ensure it does not happen again".
"We will not take any products from this site until the conclusion and satisfactory resolution of an investigation," the statement said.
'Concern' Iceland said it has "withdrawn from sale the two Iceland brand quarter pounder burger lines implicated in the study".
In a statement, the company said it noted the FSAI's findings "with concern" and "would be working closely with its suppliers to investigate this issue and to ensure that all Iceland brand products meet the high standards of quality and integrity that we specify and which our customers are entitled to expect".
Aldi said only one of its products - which is only on sale in the Republic of Ireland - was affected.
In a statement, Aldi Stores (Ireland) said: "Following notification this afternoon from the Food Safety Authority of Ireland (FSAI) of an issue in relation to our Oakhurst Beef Burgers (8 pack) we have immediately removed the product from sale and have launched an investigation into the matter."
Investigation The company said it "takes the quality of all its products extremely seriously and demands the highest standards from its suppliers".
Lidl was not immediately available for comment when contacted by the BBC.
Meanwhile, Silvercrest Foods said it has never bought horse product, and has launched an investigation into two continental European third party suppliers.