The advisory board chairman of the trust overseeing General Motors Corp.'s (GM) Adam Opel GmbH unit, Fred Irwin, said Monday that time is running out for any potential new investor to join the bidding process for the German automaker.

Speaking to reporters in Frankfurt, Irwin said all serious bidders should get access to the same information, adding that it remains difficult to predict a 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. This shouldn't be viewed as a fire sale," Irwin said.

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.

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.

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