Horse meat found in beef burgers being sold in Tesco, Lidl, Aldi and Dunnes Stores, has been blamed on suppliers in continental Europe by a processing firm.
Scientific tests found traces of horse DNA in burgers on sale in some of the UK and Ireland's leading supermarkets.
The Food Safety Authority of Ireland (FSAI) found low levels of horse in beef products sold in Tesco, Lidl, Aldi and Dunnes Stores.
Tesco's everyday value burgers contained one third horse meat
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, a subsidiary of ABP Foods, said it was pulling products from sale and replacing them with new lines.
"Following tests carried out by the Food Safety Authority of Ireland, we have been alerted to frozen beefburgers which contain porcine and equine DNA," said a spokesman.
"Although the products pose no risk to public health, Silvercrest has taken immediate action to isolate, withdraw and replace all suspect product.
"Silvercrest has never purchased or traded in equine product and has launched a full-scale investigation into two continental European third party suppliers who are the suspected source of the product in question."
According to the research by the FSAI, one sample - Tesco Everyday Value Beef Burgers - showed about 29% horse meat relative to beef content.
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.
"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," Prof Reilly said.
"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.
The FSAI analysed 27 beefburger products with best before dates from last June to March 2014 with 10 of the 27 products - 37% - testing positive for horse DNA and 85% testing positive for pig DNA.
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.
Another 19 salami products were tested but showed no signs of horse DNA.
The FSAI analysis also found traces of horse DNA in batches of raw ingredients, including some imported from the Netherlands and Spain.
Offending burgers had been sold in Tesco stores in Ireland and the UK.
Tim Smith, group technical director at Tesco, said: "Today we were informed that the Food Safety Authority of Ireland (FSAI) has found that a number of beef products they have recently tested contained horse DNA.
"These included two frozen beefburger products sold by Tesco in both the UK and Ireland. Products sold at other retailers were also discovered to contain horse DNA.
"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.
"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.
"Our customers have the right to expect that food they buy is produced to the highest standards.
"The relevant authorities have said that these findings pose no risk to public health. We understand that many of our customers will be concerned by this news, and we apologise sincerely for any distress. Our customer service team is standing by to answer any questions customers may have."
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," the company said.
In a statement, Lidl said it has 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," said a spokesman.
The Irish Farmers' Association, which is holding its AGM in Dublin, said it was very concerned at the findings.
John Bryan, president of the organisation, demanded the Department of Agriculture urgently investigate the matter.
"Nothing or no-one can be allowed compromise the high standards and reputation of Irish-produced food," he added.