Fiat says it can now exercise the option but investors had bet that GM would offer €1.8bn (£1.27bn) in cash to get out of the deal. GM maintains it has "no idea" if the two companies will end up in litigation.
"All of our options are still available to us - that has not changed - and we will pursue those options at the appropriate time," a spokesman for GM says.
He reiterates that the company believes the put option had been invalidated by restructuring moves which changed the shape of Fiat Auto. Fiat gained the right to sell its car division to GM as part of an alliance agreed in 2000. GM, whose own European operations are losing money, no longer wants to own the unprofitable Fiat unit.
It has argued the deal was voided by Fiat's decision to sell off its finance arm and halve GM's stake via a capital-raising effort.
The 2000 agreement resulted from a race between GM and DaimlerChrysler to ally with Fiat. The German firm wanted to buy Fiat outright.