Sienna Miller is the first actor to enlist in Paramount Pictures' big-budget tentpole G.I. Joe, reports Variety.

The British actress will play the film's female lead, described as a raven-haired baroness and sexy femme fatale skilled in espionage.

Stephen Sommers directs the live-action film based on the Hasbro toys, comic book and TV series.

Paramount, which has set a mid-February start date, has slotted the film for an August 7, 2009, release.

Sienna Miller's new love Rhys Ifans has asked her to marry him five times.

Sienna - who has constantly denied she is dating Rhys, despite the pair frequently being pictured together - isn't taking their romance seriously, while the "Notting Hill" actor is "besotted" with her and desperate for them to wed.

A source said, "Their romance has been a big secret for months now. Friends believe Sienna sees the relationship as a bit of fun, while Rhys has fallen in love. Rhys wants the world to know but Sienna was keen to play it down - even though she usually flaunts her romances."

"He is besotted, totally in love. He can't believe his luck. He has already asked Sienna to marry him about five times but Sienna just laughs it off. His friends are worried that she will break his heart when someone better comes along."