Harrison Ford as Indiana Jones (courtesy of LucasFilm) Enlarge Image

Shot by director Steven Spielberg, the photo shows star Harrison Ford wearing his character's trademark fedora hat for the first time in 18 years.

The original Indiana Jones trilogy, a tribute to Saturday morning B-movies, made more than $1.1bn (�560m) at the box office in the 1980s.

The fourth film, co-starring John Hurt, is due for release on 22 May 2008.

Other names lined up for the action blockbuster include Cate Blanchett, Ray Winstone, Jim Broadbent and Shia LaBeouf, who is rumoured to be playing Indiana's son.

Little else is known about the film - not even its title - but it will be set in the 1950s.

'Back in business'

The franchise began in 1981 with Raiders of the Lost Ark, in which archaeologist Jones raced around the world to find the Ark of the Covenant before it fell into the hands of the Nazis.

Rumours of a fourth chapter in the saga circulated for several years, before the film was finally announced in December 2006.

"I'm delighted to be back in business with my old friends," said Harrison Ford earlier this year. "I don't know if the pants still fit, but I know the hat will."

Producers LucasFilm recently announced that Sir Sean Connery - who played Indiana Jones' bookish father in the franchise's third instalment - would not reprise his role in the new film.