The blonde beauty, who plays Lady Rose in the hit ITV period drama, will depart the show after the upcoming fifth series in order to promote her leading role in Disney's new blockbuster adaptation of 'Cinderella'.
Bosses at the film studio have requested Lily, 25, make herself available to plug the movie's release next Easter - which would clash with filming of the sixth series of 'Downton'.
Show creator Julian Fellowes has vowed to leave the door open for Lily, however, and her character won't be unceremoniously killed off like Matthew Crawley (Dan Stevens) and Lady Sybil (Jessica Brown Findlay).
In fact, producers have urged Lily to return for the show's Christmas specials and will concoct a storyline to explain her absence from the sixth series.
A source told the Daily Mail newspaper: "Lily's young and adds balance to the family upstairs."
Lily plays the titular role in the new live action adaptation of 'Cinderella', which boasts an A-list cast including Oscar winner Cate Blanchett, 'Game of Thrones' hunk Richard Madden and Helena Bonham Carter.
The Disney film is directed by screen and stage veteran Kenneth Branagh.
An insider said: "It's a very important film for Disney and you can understand them wanting their leading lady to be available to promote it."