(CNN) -- The discovery of horse DNA in hamburgers on sale at supermarkets in Ireland and Britain is testing the appetite of meat lovers there.
The Food Safety Authority of Ireland said Tuesday that 10 out of 27 hamburger products it analyzed in a study were found to contain horse DNA, and 23 of them tested positive for pig DNA.
The horse-tainted burgers, on sale at several different supermarket chains, came from two meat processing plants in Ireland and one in Britain, the Irish authority said.
"This raises concerns in relation to the traceability of meat ingredients and products entering the food chain," it said, but noted that the findings posed no risk to public health.
In nine out of the 10 burger samples, the horse DNA was found at very low levels, the authority said, but in one sample from Tesco, Britain's largest retailer, the horsemeat accounted for about 29% of the burger.
Tesco responded by pulling from its shelves all products from the company that had supplied the dubious burgers.
"We understand that many of our customers will be concerned by this news, and we apologize sincerely for any distress," Tesco said.
The retailer is working with Irish and British authorities and the supplier to work out what had happened, it said.
Alan Reilly, the chief executive of the Irish food authority, said there was a "plausible explanation" for the pig meat finding its way into the burgers, since meat from different animals is processed at the same plants. But he said there was "no clear explanation at this time" for the presence of the horsemeat.
"In Ireland, it is not in our culture to eat horsemeat and therefore, we do not expect to find it in a burger," Reilly said. "Likewise, for some religious groups or people who abstain from eating pig meat, the presence of traces of pig DNA is unacceptable."
Many British and Irish people expressed their distaste over the revelations on social media.
Some Twitter users said they weren't surprised to hear about the questionable origins of the burgers, while others debated the ethics of eating horsemeat.
And then there were those who saw an opportunity for attempts at humor.
"Going to #tesco and expecting a beef burger. Instead you get #horsemeat . Sounds to me like foal-play," wrote Twitter user Matt Oswin under the username @BrushmanLestar.