The European Commission has warned that Germany's planned state aid for Opel may breach competition rules, raising the prospect of its sale being halted.
Brussels said there were "significant indications" Berlin pledged the 4.5bn euro ($6.7bn; £4bn) aid only if its preferred buyer for Opel was chosen.
US firm General Motors (GM) said last month it would sell Opel to Canada's Magna, Berlin's preferred buyer.
The Commission said GM should now be allowed to reconsider the sale.
Although GM picked Magna and its Russian backer Sherbank last month to buy Opel and Opel's UK brand Vauxhall, the deal has yet to be concluded.
Neither the German government nor Magna has yet given any reaction to the Commission's comments. The UK's Department for Business, Innovation & Skills also declined to comment.
However, Chris Preuss, GM's global vice-president for communications, said that if the proposed sale to Magna "couldn't pass EU regulations, we'd have no recourse but to reconsider the deal".
"Right now though we are working on a defined agreement with Magna and it's a complicated process with a lot of dialogue," he added.
"There are a lot of discussions going on at the moment, and there's a lot of detail to be ironed out between the German government and the EU."
