Matt Damon returns as former CIA assassin in latest 'Bourne' film

Actor Matt Damon returns as a troubled former CIA assassin in the latest edition of the hit "Bourne" movie series this month, reprising a role he says was physically tough to get ready for over a decade and a half since he first took it on.

"Jason Bourne" - the fifth instalment of the franchise - hits movie theatres on July 27, starring Damon as a conflicted, restless hero who resurfaces at the Greek-Macedonian border a decade after going off the grid.

At the movie's European premiere in London, Damon said it was a "lot of fun" to be back in Bourne's shoes as he returned to work with familiar faces.

But physically - preparing for a movie that features intense fight scenes and a major car chase - was challenging, the actor said.

"It was hard to get ready for it, much harder than when I was 29 - I'm 45 now, so it's whole different animal," Damon, who sat out the fourth movie in the series, told Reuters on the red carpet. "But you know, that's a good problem to have."

Asked who would win a tussle between Bourne and another legendary cinematic spy, James Bond, Damon was non-committal. But he had little doubt about who he would back as the next actor to play the MI6 spy: British actor Idris Elba.

"I like Idris - yeah, I think he's brilliant," said Damon. "I'll watch him in anything and he makes everything he's in better. He would make that franchise better."

In the latest Bourne edition - based on the novels by Robert Ludlum - co-star Julia Stiles returns as fellow operative "Nicky Parsons" while Alicia Vikander, a long-time "Bourne" fan herself, debuts as a prodigy hacker in the CIA director's team.

"The Bourne franchise was one of my absolute favourites - I'm a big fan," Vikander, dressed in a silver sequined dress, gushed on the red carpet.

"It was a real pinch-me moment when I actually saw myself, saw the film for the first time a week ago and I was like 'Oh my God, I actually got to be in one of them!’"