IG Metall labor union said Tuesday that potential investors in General Motors Corp.'s (GM) European Opel unit should approach management and labor representatives directly, critizising Germany's Economic Minister Karl-Theodor zu Guttenberg.

In a statement, IG Metall said there are "significant doubts among staff, if an economics minister should in fact pursue the sale of a company".

IG Metall added that "external sources, which are really independent" should be consulted to find and evaluate potential investors.

Zu Guttenberg met Fiat SpA (F.MI) Chief Executive Sergio Marchionne on Monday to discuss the Italian automaker's interest in acquiring Opel, which has asked for state aid to stay afloat.

Austrian-Canadian auto parts supplier Magna International Inc. (MGA) said it might take a minority stake in Opel. Various, partly contradicting reports on other possible investors haven't been confirmed yet.

Company Web site: www.opel.de

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