Prince William and Prince Harry were royal guests on the red carpet of Europe's premiere of "Star Wars: The Last Jedi" ... and the force looks very strong with these 2.

The princes suited up for the London event Tuesday night and cruised past a line of Stormtroopers before being greeted by one of the film's stars -- BB-8.

The Royal bros are more than just your typical 'Star Wars' fans, too ... they actually filmed cameos for the movie last year. Word is, they play Stormtroopers who appear alongside Tom Hardy's character while guarding John Boyega's character, Finn.

So keep a lookout for a couple 'troopers who look just a tad out of place.