German Chancellor Angela Merkel said she was skeptical that financial investor RHJ International would succeed in its bid for carmaker Opel a day before the board of General Motors was expected to announce its preference.

Merkel told the Frankfurter Allgemeine Zeitung she was very skeptical about the prospects for RHJ and reiterated her preference for a rival offer from Canadian supplier Magna .

The two are battling to buy European carmaker Opel. GM, which holds a stake in the firm, and the German government, set to provide aid, must agree on the buyer.

GM's board of directors will address the sale of Opel on Friday and aims to recommend one of the suitors, sources close to the deal have told Reuters.

German Economy Minister Karl-Theodor zu Guttenberg said he expected GM to send a signal about its preference on Friday.

He said he won assurances from other European states that have Opel plants that they would contribute to aid for Opel.

Germany, worried about 25,000 Opel jobs in Germany before next month's election, has firmed up an offer to make 4.5 billion euros ($6.40 billion) in state aid available should GM select Magna, Berlin's preferred partner.

Germany made the offer to facilitate a solution to the bidding, but would continue to work with other countries in which Opel has plants to come up with a joint solution, Government spokesman Ulrich Wilhelm told Reuters .

Trustees who oversee a majority stake in Opel -- which was propped up with German aid in May to avoid being swept into GM's brief bankruptcy -- must approve any decision.

Germany has championed the Magna bid, which also includes Russia's Sberbank , and aims to expand Opel's reach while some GM executives praised RHJ's offer, which envisages shrinking Opel to make it profitable.

The heads of Magna and Russian partner Sberbank were to meet their counterparts at GM on Thursday, a person close to the matter said.

Magna co-Chief Executive Siegfried Wolf and Sberbank CEO German Gref would hold an informal chat with GM's Fritz Henderson in Detroit, the source said, adding that no negotiations were on the agenda.

(Reporting by Angelika Gruber, Eva Kuehnen, Alexander Ratz; writing by Madeline Chambers, editing by Robert MacMillan)