The Food Safety Authority of Ireland has said that horse DNA has been found in some beef burger products on sale in supermarkets.
The meat came from two processing plants in Ireland, Liffey Meats and Silvercrest Foods, as well as the Dalepak Hambleton plant in the UK.
A total of 27 products were analysed, with ten of them containing horse DNA, while 23 of them tested positive for pig DNA.
Horse meat accounted for approximately 29% of the meat content in one sample from Tesco.
The FSAI said that the findings do not pose a risk to public health.
However, it said that it raises concerns about the traceability of meat ingredients.
Retailers have said they are now removing all implicated batches of the beef burgers.
Minister for Agriculture Simon Coveney has told the IFA AGM that he is "very annoyed" at the discovery of horse meat in burgers.
He said he learned about the FSAI survey results last night and had vets testing at the factory concerned since early this morning.
Mr Coveney said the factory concerned was in Monaghan.
He said he wanted to find out "how on earth" a test result of 29% horsemeat in a beef burger could come from a factory that does not slaughter horses or buy horsemeat.
He said he hoped to have the results in about 48 hours.
Mr Coveney said it looked like the source of the horsemeat came from additives that had been mostly imported from abroad, and added to beef to bulk up cheaper burgers at the low end of the market.
To applause from the farmers present, Minister Coveney said he would "make damn sure" this never happened again.
He said it was important to respond quickly and comprehensively to reassure people without delay.
IFA President John Bryan expressed his surprise and concern at the results of the FSAI burgers survey.
Mr Bryan said he welcomed the FSAI's assurance that there was no threat to humans from the consumption of these products.
He said the mixed meat products certainly did not come from Irish farmers, adding that whatever had happened did not take place at farm level.
Silvercrest suspects 'third party suppliers'
A spokesperson for Silvercrest Foods said the company had never "purchased or traded in equine product".
He said: "[Silvercrest] has launched a full-scale investigation into two continental European third party suppliers who are the suspected source of the product in question."
Tesco said that the brands affected in its stores are the Tesco Everyday Value brand and Flamehouse Chargrilled Quarter Pounders.
It said that it "will not tolerate any compromise in the quality of the food" it sells and is viewing the presence of horse meat in its beef products as "extremely serious".
Aldi said it has removed the Oakhurst brand of beef burgers and customers can receive a refund if they purchased any of the products.
Liffey Meats 'has identified source'
A spokesperson for Liffey Meats said the company believes it has identified the source of the contamination.
The spokesperson said: "The source of the contamination is imported ingredients and these will be replaced from other sources before production is resumed and customers are supplied.
"As confirmed by the FSAI the products concerned represent no risk to human health.
"In two of the three samples the levels as reported by the FSAI are so low as to be at the Limit of Quantification (LOQ) and for the other sample the level detected is reported as being less than 0.1%.
"We sincerely regret that any product produced by the Company would not conform to the highest specifications and sincerely apologise to our customers."