Senior Opel union boss Klaus Franz has dismissed a revised offer by RHJ International for the company as “a bad joke” and “an advertising ruse”.

His comments to Bloomberg News followed Tuesday’s announcement that Belgium-based RHJ had made a new offer for Opel and Vauxhall.

Under the revised bid, RHJ would contribute EUR300m (US$$426m) instead of EUR275m with loan guarantees of EUR3.2bn instead of EUR3.8bn repaid by 2013 instead of 2014.

The revised offer was also dismissed as inadequate by Berlin. An anonymous government official said the EUR300m bid was still too low.

Berlin has provided EUR1.5bn in short-term loans to keep Opel afloat and wants GM to accept Magna’s bid. In return, Germany would provide EUR4.5bn in guarantees for Magna.

“Germany isn’t going to change its mind on RHJ only because they improved their numbers a bit,” Uwe Andersen, a politics professor at the University of Bochum, in the western German city where Opel employs about 5,300 workers, told Bloomberg News.

Chancellor Merkel “has principal misgivings about a financial investor like RHJ, whether they pull in an industrial partner or not. She’s made her preference for Magna crystal clear.”

Merkel, who faces elections at the end of this month, has asked GM for a decision at its board meeting next week; GM has previously indicated that it would announce its decision before the start of the Frankfurt motor show on 15 September.

One option GM’s board is reported to be considering is to retain control of Opel and Vauxhall and seek financial help from all the European governments where it has factories.

But German economy minister Karl-Theodor zu Guttenberg said a restructuring of Opel that would let GM retain control hasn’t featured in the government’s negotiations with the carmaker.

On Monday, the European Commission criticised Germany for requesting a pledge from GM to keep its four factories in Europe’s biggest economy open as a precondition for state aid.

GM has parts in Poland, the UK and Spain as well as Germany. Warsaw has not received a formal request from GM for aid, the economy ministry said; London has said that any decision on the future of Opel and Vauxhall “should not be distorted by political considerations in any one country” while Spain’s prime minister Jose Luis Rodriguez Zapatero said last week that the government wouldn’t rule out giving financial help even if GM decides to retain ownership of Opel.