(Reuters) -Unionized workers who assemble Boeing's fighter jets in the St. Louis area rejected Boeing's latest contract offer on Sunday and will strike at midnight on Monday, the International Association of Machinists and Aerospace Workers union said.

The rejected deal featured a 20% wage increase – 8% in the first year, and 4% percent each in years two through four for most union members.

Boeing is bracing for the first strike in nearly three decades at its St. Louis-area defense factories after union members rejected the company’s modified contract offer. About 3,200 machinists are set to walk off the job around midnight after members voted down a deal that would’ve raised wages by 20% and boosted retirement contributions.