DETROIT- The cost to General Motors Co of keeping its European Opel unit could run $1.45 billion higher than the overly optimistic estimate prepared by the automaker in June, according to a report from a financial adviser presented to GM's board on Tuesday.

GM's board is meeting on Tuesday and Wednesday in Detroit, weighing options for Opel that include selling it to one of two bidders or attempting to raise funds to keep the unit.

Financial advisory firm KPMG said in a report presented to GM's board that the automaker would face an additional cash need of up to $6.1 billion to keep Opel.

GM had previously estimated that it would need $4.65 billion in cash to keep Opel and repay a roughly $2 billion bridge loan from the German government, the report said.

A copy of the report was obtained by Reuters.

KPMG had no comment. A GM spokeswoman could not be reached for comment.

Germany has been pushing GM to sell control of Opel to a group led by Canadian auto parts group Magna International.

Brussels-listed private equity affiliate RHJ International has also presented a rival bid.

A summary of the KPMG report said that GM had been considering a third option of abandoning the sale process and restructuring a NewOpel on its own since mid-August. (Reporting by Soyoung Kim in Detroit and Philipp Halstrick in Frankfurt; Editing by Gary Hill)