The United Auto Workers union sent a memo to Fiat Chrysler Automobiles on Tuesday notifying the auto company it will go on strike.
"In accordance with the parties' September 30, 2015 Extension Agreement, this letter shall serve as formal notice provided by the International Union, UAW and its respective Local Unions, which are signatories to one or more National and Local Collective Bargaining Agreements with FCA US LLC, of the Union's termination of the aforementioned agreements, to be effective at 11:59 p.m. on Wednesday, October 7, 2015," the letter said.
Fiat later confirmed it had received the notice. "The Company continues to work with the UAW in a constructive manner to reach a new agreement," it said in a statement.
The notice was sent five days after the UAW rejected a four-year contract deal with the automaker.
The proposed deal would have narrowed, but not closed, a pay gap between first-tier workers who currently make about $28 per hour and second-tier workers who now top out at around $19 per hour.
Fiat Chrysler has the highest share, among top automakers, of second-tier workers, 45 percent of its production workforce, and therefore has lower labor costs.
—Reuters contributed to this report.