Uncertainty over the outcome of a legal dispute over the price Fiat must pay to increase its stake in US unit Chrysler is expected to hang over the Italian carmaker's shares, a media report said.

Reuters noted that shares in Fiat hit a six-month low on Tuesday after a US union health trust said the Italian carmaker must more than double the purchasing price for a further a 3.3% stake in Chrysler Group.

Fiat, which already controls 58.5% of Chrysler, wants to pay $139.7m for the additional stake, a figure reached by using a formula set up when Fiat took control of the then bankrupt group in 2009.

But the United Auto Workers-affiliated trust said the stake was worth at least $342m in documents filed in Delaware's Court of Chancery late on Monday.

"It is difficult to make an assessment right now. We need to wait for the court ruling to make an estimate of how much Fiat would have to pay for the Chrysler stake," a Milan-based trader told the news agency.

One central issue is how to evaluate unlisted Chrysler. Fiat and the trust agreed a complex formula in 2009 to determine the value of the Chrysler stake.

"What's for sure is that low Fiat share prices are helping the Italian company as the market stock price is one of the parameters used to calculate the cost of the Chrysler stake," the trader said.

Fiat declined to comment on the lawsuit, the latest twist in a long-running dispute over the value of Chrysler between the UAW trust and Sergio Marchionne, the chief executive of both Chrysler and Fiat.

Marchionne is looking to buy the trust's entire 41.5% stake in Chrysler and the outcome of the dispute would give an indication of how much this move would cost Fiat.

Shares in Fiat opened down to hit their lowest since mid-May after the trust's request. But they later turned around and were outperforming the market by 10:40 GMT on Tuesday after Italian magistrates asked a court to unfreeze Libyan holdings in Fiat and other Italian companies.

The trust, which Fiat has sued for failing to sell it the shares, said in the documents filed to the US court that Fiat's offer was "substantially below fair market value".

Fiat, which is struggling in its core European markets, claimed a clerical error in the agreement linked Chrysler's value to Fiat's now much healthier North American unit even though the parties intended it to be the parent. The UAW trust disagreed.

"Fiat's litigious action against (the trust) reduce the likelihood of an early solution on the price of their remaining 40% stake," Barclays said in a note this week.