Workers at two of Chrysler Group LLC's largest plants voted to approve a tentative four-year labor contract between the company and the United Auto Workers union, a union official said on Monday.

The results help clear the way for Chrysler to clinch a deal with the UAW this week, just days before the smallest U.S. automaker and its majority owner Fiat SpA report third-quarter earnings on October 28.

Workers at Jefferson North Assembly Plant and Sterling Heights Assembly plants, which collectively represent more than 5,000 hourly workers, voted to ratify the deal last Friday, according to the UAW official, who asked not to be named because the information is private.

Chrysler was the last of the three Detroit automakers to reach a tentative labor contract and the deal is broadly expected to pass.

Voting on the Chrysler deal, which was announced on October 12, ends Tuesday. If the deal is rejected, Chrysler and the UAW will go to binding arbitration.

Workers at General Motors Co and Ford Motor Co have already ratified new labor contracts that will last from now through 2015.

The labor contract is the first for Chrysler's 26,000 UAW-represented workers since the company's 2009 bankruptcy, which put the company in the hands of Italian automaker Fiat. Sergio Marchionne is the chief executive of both companies.

Under Fiat, Chrysler's sales have improved, but the company and union officials have said Chrysler's financial performance is still lagging behind its crosstown rivals, GM and Ford.

As a result, the terms of Chrysler's labor deal are weaker than those at GM and Ford. For example, Chrysler workers will receive a signing bonus of $1,750 upon ratification, while GM workers received $5,000 and Ford workers got $6,000.

The terms have stoked the anger of Chrysler workers and retirees, many of whom have taken to Facebook to express their frustrations. The union has responded online by telling workers that they may be able to win back some concessions in 2015.

While GM and Ford workers voted in favor of their labor deals by a wide margin, the Chrysler vote will likely be narrower, the union official told Reuters.

(Reporting by Deepa Seetharaman and Bernie Woodall; Editing by Bernard Orr)