Up to eight different companies are reported to be preparing bids for Vauxhall/Opel.
While interest by Fiat and parts supplier Magna has been widely reported, the Financial Times newspaper says that six other firms are in the running, including Belgian car parts holding company RHJ, which owns European and Japanese car supply businesses.
The bidders have been asked to prepare firm bids for Opel and submit them for consideration over the next two weeks.
Fiat's bid is said to be more complicated than those of many of its rivals, as it also wants to take over GM’s lucrative Latin American operations as well.
Fiat boss Sergio Marchionne is also reported to have rankled GM officials, its works council, and the German political establishment by making his interest in Opel publicbefore informing them.
Klaus Franz, head of Opel’s works council, said a merger of GM Europe and Fiat could lead to a loss of up to 18,000 jobs.
"I am critical regarding the merger," he said after talks with Italian trade union representatives.
In contrast, Magna is reported to have won support from Opel’s works council, regional heads of state, and parts of its management for its bid, in which it could be joined by Russian carmaker Gaz.
Union support is vital for any buyer amid fears of plant closures at GM, which makes 1.5m cars in Europe at ten plants. GM officials have said the business needs to close at least three plants in order to survive.
GM is hurrying to secure an investor in its European arm by end-May, when it faces a US government-set restructuring deadline, and is expected to file for “surgical” bankruptcy proceedings.