Ford Motor Co. will build a new assembly plant in Mexico and sharply increase factory output from that country, representing the latest shift of investment abroad by a Detroit auto maker following the signing of a costly new labor deal.

The No. 2 light-vehicle seller in the U.S. plans to add 500,000 units of annual Mexican capacity starting in 2018, more than double what it built in 2015, according to people briefed on the plan. The plan mirrors General Motors Co.’s $5 billion investment to double Mexican capacity by 2018.

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