Potential buyers of Opel and Vauxhall must lodge their bids by the close of today.
At least three serious offers are expected to be made for GM's European arm, from Fiat, Magna and RHJ International, a Brussels-listed car parts holding company.
RHJ’s bid is being taken seriously as it has been endorsed by Klaus Franz, the head of Opel’s powerful works council. He has also welcomed Magna's bid, but is a bitter critic of Fiat, fearing a union would lead to job cuts.
Additional bids from up to three other private equity and sovereign wealth investors were still feasible but not as likely, according to the Financial Times newspaper.
They are expected to have to pay around 650 million Euros (£560 million) - although Fiat has already indicated it will offer its facilities and expertise rather than an outright cash deal.
However, GM has said it will give preference to cash bids for Opel, and Fiat's stance has angered Franz.
“We are in crisis but we still have a value,” Franz said. “Such remarks are bursting with arrogance and disrespect for the employees at Opel.”
GM, which is expected to file for Chapter 11 bankruptcy protection in the US by 1 June, has said it wants to choose a partner for Opel-Vauxhall by the end of May.