The actress employed extra security for the London premiere of 'Factory Girl' after the stalker sent her "bizarre" letters.

Sienna is so terrified of what the crazed fan might do she can't sleep.

A source told Britain's Daily Star newspaper: "Sienna has been bombarded with bizarre letters from a transvestite and has hired two security men. In his letters he talks about being obsessed with her look. He tries to follow her everywhere.

"The poor girl hasn't been sleeping properly because she has been so scared. Things just got to the point that she just had to take extra care of herself."

Sienna's stalker is a transvestite called Peter, who wears women's wigs, high-heeled stilettos, fake fur coats and dresses.

Peter is also said to be obsessed with the 'Factory Girl' movie, which stars Sienna as Edie Segdwick, pop artist Andy Warhol's muse.

Sienna, 25, is said to be so concerned about her safety, she has upgraded the security system in her West London home.