July 27 (UPI) -- Several thousand Boeing union workers at three St. Louis-area plants who build fighter jets are planning to go on strike after rejecting a proposed contract Sunday that would pay an average of than $100,000 per year.

Members of the International Association of Machinists and Aerospace Workers at Boeing factories in St. Louis and St. Charles in Missouri and Mascoutah in Illinois voted against the new contract that included a 20% wage increase over four years.

The contract for District 837 members will expire at 11:59 p.m. CDT at which point there is a seven-day cooling-off period before a strike could start.

In all, there are 16,000 employees at the three locations, according to St. Louis Business Journal Research.

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