Michelle Dockery, who plays Lady Mary in the global hit, posted a photo on Instagram with the caption, "And...we're off."
The film will reunite the Crawley family on the big screen. Series creator Julian Fellowes wrote the screenplay and will produce.
The primary cast members are all set to return, but the plot remains very hush-hush.
Over six seasons, "Downton Abbey" aired in at least 150 countries and set a record for non-U.S. television shows with 69 Emmy nominations.
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