David Benioff and D.B. Weiss, who adapted George RR Martin’s fantasy novels for TV, had previously been announced as working on a film in the Star Wars franchise.
Disney had previously said Rian Johnson, who directed 2017’s The Last Jedi, was working on a new Star Wars film.
It was unclear which would come first.
Benioff and Weiss’ movie will be the next instalment after The Rise Of Skywalker hits cinemas in December.
The film, directed by JJ Abrams, will close the Skywalker trilogy.
Benioff and Weiss have been praised for their stewardship of Game Of Thrones.
Despite its success the fantasy drama was edged in the ratings by the popular sitcom The Big Bang Theory last week.
Both shows are approaching their finales.
The Big Bang Theory, made by CBS, drew 12.6million live viewers compared to HBO’s Thrones which had 12.5million, according to the Nielsen data company.
It was the second week in a row the sitcom had attracted more viewers, whose 12-season run ends with a 16 May finale episode.
The past week was still a big win for Game of Thrones as the latest episode was the most-watched of any show in HBO history.
The network is also available in far fewer homes than CBS and other rivals.
Game of Thrones’ penultimate episode “The Bells” drew 18.4million viewers if reruns later that night and streaming services views are included.
The figures breaks the record of 17.8million set an episode earlier in the season.
Next week’s series finale, whose title has not been revealed, is likely to crush those numbers.