Actress Rachel McAdams lost her nerve when she turned up to pose naked for a new Vanity Fair photospread - and it cost her a spot on the cover of the provocative new Hollywood issue.

THE NOTEBOOK star was invited to pose with British beauty Keira Knightley and Scarlett Johansson by the issue's artistic director, designer Tom Ford, but pulled out at the last minute.

Ford explains, "She felt that she'd be fine with it and then she got there and she realised that she really wasn't, and that was fine... She just asked if she could be excluded from the cover."

Ford himself replaced McAdams for the Annie Liebovitz cover shoot - and posed with naked Knightley and Johansson, who were also nervous about baring all to the world.

He adds, "I said to them, 'When you're 70 you're gonna look back and say, 'Thank God for this picture - look how amazing I was.'"