(Reuters) -Unionized workers who assemble Boeing's fighter jets in the St. Louis area rejected Boeing's latest 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 Co. has offered a new contract to the union representing some 3,200 factory workers in the St. Louis area, including the same 20% raise that members resoundingly rejected in an earlier deal while voting to strike.