Ireland's agriculture department has suspended production at a small meat plant after it said on Friday that it discovered the plant had sent horse meat to the Czech Republic incorrectly labelled as beef.
Europe's horse meat scandal, which has prompted product withdrawals, consumer concerns and government investigations, erupted last month after tests carried out in Ireland on beef products supplied to the likes of Tesco.
Irish producers and investigators have blamed Poland for raw materials that saw as much as 75 percent horse DNA included in its products - a charge Polish food safety agencies deny - but the department said the latest find was down to mislabelling.
"I am seriously concerned about this development and the Gardai (police) have been fully appraised of this development and are working closely with my department," agriculture minister Simon Coveney said in a statement.
"The issue here is one of mislabelling and that will be the focus of the investigation."
B&F Meats, a southern Irish plant that debones beef and horsemeat and dispatches it via a UK-based trader, had sent some horsemeat to a single customer using a Czech label that, when translated, referred to beef, the department said.
Ireland, which holds the EU presidency, called a meeting of European ministers earlier this month that proposed increased DNA testing to assess the scale of a scandal.
Coveney said he has arranged a special debate in Brussels on Monday during a meeting of Europe's Council of Agriculture and Fisheries.