Fiat Group may have to wait until 2015 for a trial over the value of some Chrysler Group shares owned by a union healthcare fund, further delaying a plan to combine the two companies.

Lawyers for a United Auto Workers trust fund are asking Delaware Chancery Court Judge Donald Parsons to hold a January 2015 trial on the terms of a stock agreement covering at least 54,000 Chrysler shares, according to court filings. Fiat is seeking a May 2014 trial, according to the filings.

Fiat-Chrysler CEO Sergio Marchionne wants to buy the shares as part of an effort to transform Fiat and Chrysler into a global auto producer. The trial will help determine how much the shares will cost, and that decision will clear the way for a complete integration of the companies.

“Delaying the court hearing on the options is not good news,” said Philippe Houchois, an analyst at UBS in London, who estimates Fiat may pay $4 billion euros to acquire full control of Chrysler.

On Wednesday, the trust’s lawyers said in a letter filed with the court in Wilmington that the union’s proposal “would bring this matter to trial within a reasonable time that would allow the parties and this court to proceed at a suitable pace.”

Parsons refused in July to honor Fiat’s request to set a value on the shares without a trial. The judge did rule in Fiat’s favor on several claims affecting the value of the shares held by the trust.

Fiat bought a stake in Chrysler in June 2009 after the U.S. carmaker sought bankruptcy-court protection from creditors.

Raymond DiCamillo, a Wilmington-based lawyer representing Fiat, didn’t immediately return a call after regular business hours seeking comment on the trial-schedule proposals. Gualberto Ranieri, a Chrysler spokesman, also didn’t immediately return a call seeking comment on it.

The trial will focus on the agreement’s techniques for valuing the first segment of more than 270,000 shares the trust received when Chrysler emerged from bankruptcy in 2009.

Marchionne has spent the past four years seeking to unify the companies so they can better compete with Toyota Motor, General Motors and Volkswagen Group. A fully integrated automaker would feature the mass-market Fiat, Chrysler, Jeep and Dodge brands, along with the high-end Maserati and Ferrari lines.

Marchionne has said the Delaware case will help set a value on the shares. Fiat wants to acquire the UAW’s entire Chrysler portfolio of 676,000 shares, or 41.5 percent of the carmaker, as part of its unification effort.

The UAW’s Retiree Medical Benefits Trust provides healthcare services to more than 800,000 retirees and their dependents, according to the trust’s Web site. It’s the largest nongovernmental buyer of retiree health services in the U.S.

Fiat’s review of the first block of union shares available for purchase pegged their value at about $140 million, according to court filings. The union trust countered its calculations found the block was worth as much as $342 million.

Fiat’s lawyers are seeking to have pretrial information exchanges done on an “unreasonable” schedule, given the number of documents to be turned over and depositions to be conducted, Ken Nachbar, one of the trust’s lawyers, said in a letter to Parsons.

He said that some of the witnesses in the case, including accountants, are based in Europe and may have to be compelled to testify. The case involves “complex accounting issues” that will take time to explore, Nachbar said.

At trial, Parsons will hear evidence on the call-option agreement’s formulas for valuing the trust’s shares to help set an acquisition price.

The formulas are based, in part, on Chrysler’s reported earnings before interest, taxes, depreciation and amortization for the four most recent financial quarters, minus the carmaker’s “net industrial debt” as listed in its most-recent financial statement, according to court filings.

The pact, created to avoid disputes over the value of the trust’s shares, “has led to substantial disagreement” over what the stock is worth, Parsons said in his July 31 ruling. “The value of this dispute potentially could exceed a billion dollars,” he said.

The case is Fiat North America v. UAW Retiree Medical Benefits Trust, 7903, Delaware Chancery Court (Wilmington).