Miss Knightley, 21, spoke out after her appearance in a low-cut Gucci dress at the London premiere of her latest film this week prompted concerns that she has lost too much weight.

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The Pirates Of The Caribbean actress said her grandmother and great-grandmother suffered from the illness - but insisted she has not followed in their footsteps.

She claimed to be surprised when journalists asked her about her eating habits.

'They said to me yesterday, "How does it feel to be called anorexic?" and I had no idea that I was. I can safely say that I'm not,' she told a press conference yesterday.

Then she went on: 'I've got a lot of experience with anorexia - my grandmother and great-grandmother suffered from it, and I had a lot of friends at school who suffered from it, so I know it's not something to be taken lightly and I don't. But I don't have it, I am very sure of that.'

However, she said she welcomed debate about the illness. She said: 'In a way it's good that it's out there and people are talking about it. It's quite interesting because it's normally high-achieving women who suffer from it because I guess they are control freaks.

'I'm not saying there aren't people in the film industry that suffer from it, because I am sure that there are, but I am not one of them.' She went on to urge young girls not to think of her as a role model.

'I reject being a role model for a young person,' she said. 'I think it's completely ridiculous, the idea that you put someone up there and say, "That's how you should behave'.

'I make mistakes like everyone else. But having a character as a role model, that's okay - if people look up to a character like Elizabeth in Pirates then that's great.'

The previous night, at the premiere, the actress wore a gold dress slashed to the waist which revealed an extremely slender frame.

The outfit caused some onlookers to wonder if the British starlet had bowed to Hollywood pressure to be super-skinny.

But Miss Knightley said: 'Whatever people say about my weight they are all wrong. Hollywood is all about the way you look and I don't think that's the healthy thing for anyone. But if you're strong and comfortable with yourself then you're going to be fine.'