Actress Keira Knightley has donated the emerald green gown she wore in 'Atonement' to be auctioned for charity.

Bidding for the Thirties-style silk gown has already reached US$6,000 on the Clothes Off Our Back website.  Proceeds will be donated to The Children's Charity of Southern California.

The gown was voted the "best film costume of all time" in a survey by British InStyle magazine and Sky Movies earlier this year, ahead of Marilyn Monroe's white dress from "The Seven Year Itch" and Audrey Hepburn's little black dress from "Breakfast at Tiffany's".

The auction closes on March 1.