The satement was the European Commission's sharpest criticism since Berlin promised 4.5 billion euros ($6.6 billion) in aid to Opel if a group led by Canada's Magna buys a majority stake in the company from General Motors.
GM has agreed to sell a 55% stake in Opel to Magna and Russian partner Sberbank. Britain and Belgium, which have Opel plants, fear the new owner will want to protect jobs in Germany more than elsewhere because of the aid pledge.
Britain entered the fray when business secretary Peter Mandelson said Magna's restructuring plan would be open to political intervention, too expensive and punish productive plants.
The European Commission, the 27-country EU's competition watchdog, has repeatedly said it will review any state aid to ensure it adheres to strict EU rules.
European Competition Commissioner Neelie Kroes likened protectionism to a "raging wildfire almost impossible to control".
"We cannot accept one government bribing companies in order to steal or end the jobs of another," she said in the text of a speech for delivery at an antitrust conference at Fordham University in New York.
"We cannot accept companies becoming addicted to aid. Such behaviours are a recipe for a trade war and poverty - not a way out of this recession," she said, adding the car industry must carry out reforms.
EU countries to debate Opel aid doubts on 7 October
Kroes forced the French government in February to back down after it proposed a 6-billion-euro state loan for Renault and PSA Peugeot-Citroen in return for an unwritten pledge not to close their French plants during the loan period.
"We would not hesitate to halt other problematic aid if it comes to our attention. For that reason I am currently examining carefully the conditions for the rescue by Germany of GM's European arm, Opel," she said.
The European Commission and officials from countries hosting Opel factories will discuss on 7 October Germany's state aid plan.
Gunter Verheugen, the EU Commissioner in charge of enterprise and industrial policy said "what we will do on the 7th of October is to decide on the basis for a decision. The decision will be whether there will be state aid and who is going to pay," Verheugen told a meeting of EU ministers in charge of competitiveness on Thursday (24 September).
(EurActiv with Reuters)
