Newly single Sienna got into the festive spirit this week as she helped Santa turn on
the lights at Cartier's New York boutique.[Getty Image]

"It's a mutual decision," a pal of Jude said of the break-up, "They have definitely come to the end of the road. They tried to make it work but it failed." The friend's comments have apparently put the final nail in the coffin of the on-off relationship that made headlines around the world. "Fundamental differences" have been blamed for the end of their three-year relationship.

New-York born Sienna, 24, became engaged to her handsome Brit beau on Christmas Day 2004, but just months later the actor's affair with his children's nanny Daisy Wright was revealed. Jude made a public apology and the pair tried to get their roller-coaster romance back on track, but it wasn't to be.

"There isn't another person involved, but it isn't helped by Jude's devotion to his kids," says the pal. Jude, 33, and his ex-wife Sadie Frost enjoy a "very friendly relationship for the sake of the kids" and this is thought to have caused tension between Sienna and her one-time fianc¨¦.

The pretty blonde star won't have too much time to dwell on her love-life, however, as her career is currently going stratospheric. She's soon to hit screens as Edie Sedgwick in Andy Warhol biopic Factory Girl and her performance is already generating buzz among the Hollywood set. Perhaps appropriately so, as the actress says she gave the part her all.

"The more I read - and I read everything - the more I became really intrigued by her," says Sienna. "I know I couldn't have done any more. I couldn't have learned any more, studied any more, given any more emotionally."