The advisory board chairman of the trust overseeing General Motors Corp.'s (GM) Adam Opel GmbH unit, Fred Irwin, said Monday that it might take longer than initially planned to finalize the German automaker's sale.

"This shouldn't be viewed as a fire sale...All serious bidders should get access to the same information," Irwin said at a press briefing in Frankfurt, adding that it remains difficult to predict a concrete timeframe for a final decision.

Irwin said that deals like that usually take between seven and nine months to be finalized, adding that he hopes the deal can be completed within his six-month tenure.

"But it could take longer than that if needed," Irwin said.

The president of GM Europe, Carl-Peter Forster, said earlier this month he expects a definitive agreement to be reached by mid-July, with the closing of the deal to be completed by September.

Irwin said the time is running out for any potential new investor to join the bidding process in addition to the companies who have signalled their interest so far.

Irwin said Opel has cash reserves in the "low three-digit million euro range." He declined to be more specific.

Last month, GM signed a memorandum of understanding for a sale of a majority stake in Opel with Canadian auto supplier Magna International Inc. (MGA), which has launched an offer backed by Russia's Sberbank (SBER.RS) and automaker GAZ (GAZA.RS). But Irwin noted that this isn't a legally binding agreement.

The agreement with Magna is backed by EUR1.5 billion in bridge financing provided by the German government. As part of this agreement, a trust led by Alfred Hagebusch, a Frankfurt-based lawyer for German law firm Wellensiek, and GM Europe vice president Eric Stevens has been established, which is overseeing Opel.

German Economy Minister Karl-Theodor zu Guttenberg said in a interview with Austrian paper Der Standard published Sunday that nobody can rule out that GM's talks with Magna might turn sour. In his view, Magna's role as supplier to competing car makers represents a risk to the planned deal.

Italian automaker Fiat SpA (F.MI) and China's Beijing Automotive Industry Holding, or BAIC, have voiced interest in Opel, but the status of their respective bids remains unclear.

Details of BAIC's bid haven't been released. Fiat Chief Executive Sergio Marchionne last week reiterated the company's interest in Opel, but said that Fiat won't come up with a new offer.

Opel's supervisory board member and labor representative Armin Schild told Dow Jones Newswires Friday that the powerful labor union IG Metall still prefers Magna as new investor.

German weekly Focus earlier Monday quoted a spokesman for RHJ International SA (RHJI.BT) as saying that it is out of the bidding process.

Company Web site: www.gm.com

-By Christoph Rauwald, Dow Jones Newswires; +49 69 29 725 512; christoph.rauwald@dowjones.com