Horse DNA has been found in beef stored at three cool stores in Poland.
The country’s General Veterinary Inspectorate said it had found three tainted samples from 121 tested.
Further testing is under way to confirm the percentage of horse meat in the beef.
Eighty more samples have yet to be examined.
Deputy National Veterinarian Janusz Zwiazek said the samples came from cold storage facilities at processing plants in central Poland. The meat had been sourced from suppliers in Poland and abroad.
Polish lawmakers have been angered over allegations that beef sourced from the country may have been responsible for the contamination of processed beef products in several European countries.
These are the first tests in an extensive programme in Poland to reveal the presence of horse meat.
The horse-meat contamination scandal erupted in mid January, when the food watchdog in the Irish Republic announced results on the testing of beef burgers, which revealed a significant number of tested products were tainted with horse meat.
In the worst case, one burger proved to be 29 per cent horse meat.
Testing throughout Europe has since revealed similar contamination across a range of processed beef products.