The channel had kept its audience of almost 12 million waiting longer than usual to confirm a further run of the show, scripted by Julian Fellowes.
The eight-part run next year will open and close with feature-length episodes as the tales of the Crawley family and their servants in the early 1920s continue, with filming once more at Highclere Castle.
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There will also be another Christmas edition.
This year's two-hour festive episode sees the family head to a Scottish estate – filmed at Inveraray Castle – to visit cousins, against a backdrop of deer stalking in the Highlands.
But many of their staff remain at Downton, and further dramas unfold in the family's absence.