The revised Magna bid submitted to General Motors for Opel "contained elements around intellectual property and our Russian operations that simply could not be implemented", the automaker's top negotiator reported through the somewhat unusual medium of a website blog.

Acknowledging the Magna pitch was clearly preferred "by several politicians and the labour bench [Opel's unions and works councils]", GM group vice president John Smith said the bid presented to GM varied from the negotiations in previous weeks.

"GM has partners in other parts of the world who have joint ownership of these assets…we simply could not execute the deal as submitted," he said.

Discussions with Magna continued but the only answer to timeline queries was "as soon as possible."

"To say it's been an intense negotiation would be a dramatic understatement," he wrote in a message that appeared directed mainly at GM employees.

"Despite media reports to the contrary, GM has NOT specified its preference for a bidder," Smith emphasised.

"Both bids being developed bring both opportunities and challenges…both bids vary in how they would be implemented. But we remain fully open to reaching a satisfactory conclusion with either bidder."

He noted both offers included strong links to GM's global product development system, seen as essential for Opel/Vauxhall to compete, and said the completed bid from RHJI was simpler and would be easier to implement, as well as needing less government financial support.

"This remains a reasonable and viable option to be considered as the very difficult issues around the Magna negotiations continue to be worked," Smith wrote.

He added that he nonetheless hoped to have a presentation ready for the Opel Trust and the German government by the end of September.