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About 64% of the engineering unit and 72% of the technical unit voted down Boeing's four-year proposal; the earliest a strike could occur is Oct. 7

John van Hasselt - Corbis / Getty Images

Boeing $BA's engineers and technical workers voted to reject the planemaker's four-year contract offer, with members of the Society of Professional Engineering Employees in Aerospace also authorizing the union's negotiating team to call a strike when the contract expires in October.

The professional unit — engineers and scientists — turned down the offer by a 64% margin, while the technical unit, encompassing technicians, analysts, and planners, did so by roughly 72%, according to the union. Strike authorization passed by wider margins: 88% of the professional unit and nearly 90% of the technical unit voted yes. SPEEA represents about 17,000 workers and is Boeing's largest white-collar union.