Keira Knightley has said her current film project Atonement will be "grown up" compared to Pirates of the Caribbean.

The movie of the Ian McEwan novel will see the star play student Cecilia, who falls in love with doctor Robbie, played by James McAvoy.

In one scene Robbie writes an explicit letter to Cecilia and they later make love.

Knightley explained of her character: "She's a grown-up and she does grown-up things.

"I was 17 when I started playing Elizabeth in the Pirate films and they were fun, but very much meant for families. Now I've grown up and so have the roles."

The movie is due for UK release in September.