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.

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.

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

German weekly Focus earlier Monday quoted a spokesman for RHJ International SA (RHJI.BT) as saying that its 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