Belgian finance investor RHJ International raised the stakes in the ongoing bidding war with rival Magna for GM's Opel, a spokesman for RHJ said on Tuesday, confirming a report in a German paper.

In a letter to GM's management, RHJ said it is now offering 300 million euros ($430.8 million) in cash for a 50.1 percent stake in Opel and would ask for only 3.2 billion euros in state aid that it pledged to then pay back by 2013, a year earlier than initially planned.

No other changes had been made to the bid, the spokesman said, adding that no cuts to investments or to product plans would need to be made as a result of RHJ's new bid arithmetic.

Previously it had offered 275 million euros in cash and requested 3.8 billion euros in government loan guarantees.

The news of a formal improvement was initially reported by German daily Rheinische Post, citing a copy of the letter to GM it had obtained.

(Reporting by Christiaan Hetzner)