(Reuters) -United Auto Workers (UAW) union members at General Motors (NYSE:GM)' Flint assembly plant in Michigan have narrowly voted against a proposed contract with the U.S. automaker, the local chapter said.
In a Facebook (NASDAQ:META) post on Thursday, the UAW Local 598 said 51.8% of votes cast were against the proposed deal.
GM said it would not comment during the ratification process.
Shares of the company fell about 1% in choppy morning trade to hit a more than three-year low of $26.34.
The vote signals that approval of the deal, which is set to raise costs significantly for GM, is not guaranteed.
Workers at the company's other plants are expected to vote on the agreement in the coming weeks.
Union workers are voting on contracts from each of Chrysler-owner Stellantis, General Motors and Ford Motor (NYSE:F), after the first coordinated strike against Detroit's Big Three automakers.
The vote at the Flint assembly plant, which manufactures the Silverado heavy duty pickup truck, comes after the Detroit Three automakers and the UAW reached tentative deals over the last few weeks to end a costly strike following marathon negotiations.
The UAW's new agreement, which covers 46,000 workers overall at GM, grants a 25% increase in base wage through April 2028 and will cumulatively raise the top wage by 33% compounded with estimated cost-of-living adjustments to over $42 an hour.
Automakers had previously been slashing costs and navigating a bumpy road to manufacture EVs and catch-up with market leader Tesla (NASDAQ:TSLA). However, lower margins on those vehicles have deterred them from accelerating the move.
GM in October pulled its full-year profit forecast and postponed a $4 billion electric truck plant in Michigan.
Separately, the UAW said Volvo Group-owned Mack Trucks has informed the union that the Oct. 1 offer to striking workers was its "last, best and final".
Voting on the offer has been tentatively scheduled for Wednesday, the union said .