King received a phone call at 0730 BST on Thursday informing him that his partner was on her way to hospital.
The Tottenham player was due to be on the bench against Portugal but flew back to England at 1000 BST.
The 23-year-old, who started England's match against France, would have returned to Portugal for the semi-finals had England go through.
But King's and England's tournament is over following Portugal's penalty-shoot victory in Thursday's quarter-final.
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