BERLIN – General Motors Co.’s planned sale of Opel to Magna International Inc. was thrown into doubt late Friday after European Union regulators expressed concerns over Germany’s offer to support the deal with Euro 4.5 billion ($6.72 billion) in state aid.

Germany made its financing offer contingent on Magna’s winning the bidding for the European auto maker, a precondition that “would be incompatible with EC (European Commission) Treaty state aid and internal market rules,” European Competition Commissioner Neelie Kroes wrote in a letter to German Economics Minister Theodor zu Guttenberg reported by The Wall Street Journal.

Kroes said in her letter that Germany should offer the other Opel bidders the same financing guarantees as Magna and its Russian partners – auto maker OAO GAZ Group and state-controlled OAO Sberbank – and that GM should be allowed to reconsider those bids.

Kroes’s letter, though not a formal ruling, suggests that the commission, the EU’s executive body, may seek to block the deal if it is submitted for approval in its current form. That could force Germany to either drop its plans to finance the deal, which may force Opel into bankruptcy, or accept another bidder. Though GM and Magna agreed in September to a deal in principle, an agreement has yet to be signed.

In addition to Magna, Belgian private-equity group RHJ International Inc. made a bid for Opel, an offer that GM’s senior management initially favored. Yet once it became clear that Germany’s offer of financial aid – the key to completing a sale of the loss making car maker – was only open to Magna, GM’s board agreed to the deal. Berlin also opposed the idea of GM keeping and revamping Opel itself, one option GM’s board also wanted to consider.

It is unclear if RHJ, which couldn’t be reached tocomment, or other parties are still interested in bidding for Opel.

Under GM’s preliminary agreement with Magna, the Canadian auto-parts maker would acquire a 55 percent stake in Opel and its U.K.-based sister company Vauxhall. Opel employees would hold 10 percent, while GM would retain the remaining 35 percent. Opel and Vauxhall have been losing money for a decade, despite repeated restructuring efforts.

Kroes’s letter arrived just as Germany had been trying to assuage concerns about its aid package by trying to include other European countries with Opel operations in the deal. By reaching out to other countries, Germany was hoping to counter suggestions that its aid offer was based on a quid pro quo arrangement with Magna to keep German plants in operation.

On Friday, the German government said it had secured commitments from Austria and Poland to contribute toward the �?�4.5 billion financing package and said it hoped to reach a similar agreement with the U.K. soon. The Germans had hoped to have the deal signed between GM and Magna early next week.

Kroes had warned previously that she would oppose a deal that appeared to make state aid conditional on preserving local plants and jobs.

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