Food supplier Comigel has its headquarters in Metz, France and a processing plant just over the border in Luxembourg.
Tesco and Aldi have withdrawn a range of ready meals produced at the factory.
Comigel alerted Findus and Aldi that their products "do not conform to specification" on Monday.
Boss Erick Leharge, 53, has so far refused to comment.
The EU is urging member states to carry out tens of thousands of tests in the wake of the horsemeat scandal - but who will foot the bill?
Romanians are smarting at accusations that abattoirs there put horsemeat into the food chain.
Poland's deputy chief veterinary officer says his investigation shows the country's meat products are "genuine beef" and alleges fraud.