Prospective Vauxhall and Opel buyer RHJ International has raised its bid for the companies.
The news comes on the eve of the latest GM board meeting at which the sale will be discussed.
RHJ confirmed it had informed GM in a letter that it would now offer 300 million euros (£264 million) in cash for a 50.1 per cent stake in Opel, 25 million euros (£22m) more than it first bid.
Moreover, the company said it would ask for only 3.2 billion euros (32.8bn) in state aid from Germany and other European countries where Opel has factories, £528m less than before. It also offered to pay the loans back by 2013, a year earlier than before.
"RHJ had already stated it was trying to optimise the financial aspect of its offer and it will continue to make every effort wherever possible to reduce the impact on German and European taxpayers without making any new cuts in employment, planned investments or anything else," said an RHJ spokesman.
GM is expected to meet today to discuss the rival offers from RHJ and Magna.