Princes William and Harry are due on the red carpet for the European premiere of "Star Wars: The Last Jedi," in which the royal siblings reportedly make a cameo appearance.
Royal officials refuse to comment on reports that the princes played stormtroopers in the sci-fi saga when they visited the film's set in April 2016.
But star John Boyega has said the royal duo filmed a scene during their visit to London's Pinewood Studios, though it's unclear whether it made the final cut.
Stars of the film including Boyega, Daisy Ridley, Adam Driver and Mark Hamill are expected at Tuesday's gala screening at London's Royal Albert Hall.
The screening is a benefit for the princes' charity, the Royal Foundation.
The film had its world premiere Sunday in Los Angeles.
A previous version of this story did not make clear that the royal set visit was in April 2016.