Tesco, along with a number of other supermarkets, has removed certain brands of frozen beefburgers from its shelves in the UK and Ireland after they were found to contain horsemeat.
A study examining the authenticity of a number of beefburger, beef meal and salami products available from retail outlets in Ireland found horsemeat accounted for approximately 29% of the content in one sample of Tesco Everyday Value Beef Burgers.
Tests on beef products sold in Lidl, Aldi, Iceland and Dunnes Stores uncovered low levels of horse DNA.
Authorities have said there is no threat to public health, but the issue is one of consumer confidence and shoppers being able to trust that what they are eating is what was stated on the label.
Tim Smith, group technical director at Tesco, said: "We immediately withdrew from sale all products from the supplier in question.
"We are 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.
"We understand that many of our customers will be concerned by this news, and we apologise sincerely for any distress."
The Food Safety Authority of Ireland (FSAI) tested for the presence of horse and pig DNA.
A total of 27 beefburger products were analysed, with 10 (37%) testing positive for horse DNA and 23 (85%) testing positive for pig DNA.
Some 31 beef meal products including cottage pie, beef curry pie and lasagne were also analysed. Twenty-one were found to have pig DNA, while all were negative for horse DNA.
All 19 salami products analysed tested negative for horse DNA, but traces were detected in batches of raw ingredients including some imported from The Netherlands and Spain.
The beefburger products which tested positive for horse DNA were produced by two processing plants in Ireland, Liffey Meats and Silvercrest Foods, and one in the UK, Dalepak Hambleton.
They were on sale in Tesco, Dunnes Stores, Lidl, Aldi and Iceland. In nine of the 10 beefburger samples from these retailers, horse DNA was found at low levels.
The tests found horse DNA 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%, and 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.
Sky's Health and Science Correspondent Thomas Moore said: "These days, meat is traded around Europe. There is a suggestion that this horse meat didn't actually come from Britain or Ireland and it may well have been imported from Spain or Holland.
"In some parts of the continent, horse meat is eaten and is perfectly normal."
The FSAI said it was working with the Department of Agriculture, Food and the Marine, as well as the processing plants and retailers involved.
It said the retailers had pledged to remove all implicated batches from their shelves immediately. In addition, Silvercrest Foods was withdrawing all products from sale and replacing them with new ones.
Aldi said it was conducting its own investigation. "We have sought information from one supplier, Silvercrest, which is dealing directly with the FSAI on the issue that has been raised," it said.
Lidl said it had taken the decision to remove all implicated products from sale pending a full investigation.
"A refund will be provided to customers who wish to return affected products," a spokesman said.
Professor Alan Reilly, the chief executive of the FSAI, said although consumers need not worry, the findings did raise a number of concerns.
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. Consumers who have purchased any of the implicated products can return them to their retailer.
"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. Likewise, for some religious groups or people who abstain from eating pig meat, the presence of traces of pig DNA is unacceptable."
:: The FSAI operates an advice line on 1890 33 66 77 from 9am-5pm.