Filming has begun already on the project, which features the same cast as the TV series that ended in 2015.
Why Downton Abbey is forever YorkshireCreator Julian Fellowes will write the script for the movie, which will be made by Universal Studios and directed by Brian Percival.
The fictional Downton Abbey is located in North Yorkshire, and the characters make regular references to towns such as Thirsk, Ripon and Easingwold.
However, Highclere Castle in Hampshire was used as the location for the house and most of the six seasons were shot in London, Oxfordshire, Northumberland and Scotland.
The film is expected to be set in the year 1926, when the final season finished.
The real reason Downton Abbey was set in YorkshireDame Maggie Smith, who plays the Dowager Lady Grantham, had previously suggested it was unlikely she would ever reprise the role, as it would be implausible that the elderly matriarch would still be alive.
The period drama, which followed the fortunes of Lord and Lady Grantham, their three daughters and their household, achieved a rare feat for a British show by breaking the US market.