Fiat Auto SpA and its equity alliance partner General Motors Corp. will use their last civil avenue to decide the legitimacy of a “put option” in their pact that allows Fiat to force GM to acquire the 90% of Fiat Auto it doesn’t own.

After a lengthy meeting between GM Chairman Rick Wagoner and Fiat SpA CEO Sergio Marchionne, the two leaders were unable to reach an agreement over how to proceed with their cooperation and whether the put option was valid.

Failure to reach an understanding has forced the two parties to activate the mediation process set out in the master agreement reached between the auto makers in 2000.

Under the terms of the mediation, the chief executives of the two auto makers will meet within 20 business days of initiation of the mediation clause. The meeting will be held “in a neutral setting in person to attempt in good faith to resolve such matter,” the agreement says.

If the two sides still are unable to reach an accord they can resort to legal remedies 10 days after the meeting.

Fiat is said to be set on disposing of its ailing auto business, which has about €8 billion ($10.6 billion) in debt and is expected to post a loss of approximately €800 million ($1.06 billion) this year.

Reports indicate the Italian auto maker may be willing to revise the put option if GM agrees to a cash infusion in Fiat Auto. Fiat also would like the ability to set up partnerships with other auto makers outside of the GM group.

GM, meanwhile, contends the put option is no longer valid, since Fiat conducted a recapitalization of its auto unit, which changed its ownership structure.

According to the timetable outlined by the master agreement, the earliest mediation could begin is late January, around the same time Fiat can exercise the put option, which goes into effect on January 24.