Keira Knightley has joked it's no wonder Jude Law wore a hat in their new movie, as he looked so bad.

The British actress took pity on her handsome co-star while making the period drama Anna Karenina, because his costume disguised his good looks. Keira giggled that his transformation into a 19th century character was so realistic that even the crew were fooled into believing he was less than appealing.

"No wonder he kept that hat on. Poor f**ker. I'd get to the end of the day, take my make-up and wig off, and I'd look sh*t," Keira told Harper's Bazaar magazine.

"He'd take the make-up and wig off and look f**king brilliant. And everyone would say, Oh yeah! Good-looking man underneath!'"

In the movie, Keira plays aristocratic socialite Anna in 19th century Russia. She is unhappily married to loathsome government official Alexei Karenin, who is portrayed by Jude.

Despite claiming she needs cosmetics to look her best, Keira is regarded as one of the world's most beautiful stars.

However, the 27-year-old brunette has struggled to accept herself as she's got older.

"I hated late teens, I hated early twenties, all of that, I think a lot of people do. I think I'm one of those people that always would have done and it didn't really matter where I was in life," Keira explained. "I was totally uncomfortable in everything."

The actress has felt more settled since finding love with rocker James Righton. The newly-engaged star is due to get married later this year and is ready for the commitment.

"All of a sudden 25 hit, and I was like, Ah, I think I'm all right,'" Keira admitted.

"And actually, after 25 it's been pretty good. I definitely feel like I've been around the block a couple of times, and that's great."