Much has been made of Keira Knightley’s slender frame and relatively flat chest.

Indeed, Keira herself has spoken openly of not having breasts, but ‘pecs’, and said that she would die for a curvy body like Scarlett Johansson’s.

But in an extraordinarily revealing interview, the actress has told of how American audiences view her rather differently.

The 21-year-old star of the Pirates of the Caribbean films has spoken of her bemusement at being surgically enhanced by computer trickery in promotional material publicising her films in the US.

She says studio executives and magazine publishers have decided that instead of presenting her as she really is, she should be seen as a lot bustier in case her flat chest ‘turned people off’.

The actress says her breasts were digitally boosted for the advertising campaign for her 2004 movie King Arthur.

She also claims magazine publishers in the US ban stars from appearing on their front covers unless they have at least a C-cup size, or are willing to be digitally enhanced to make it appear as if they have.

Keira, who played Guinevere in King Arthur, said her normal breasts were blown out of all proportion and looked droopy on the advert to promote the film.

‘Those things certainly weren’t mine,’ she told a US magazine yesterday.

‘I remember we had an interesting discussion when they said, “We want to make them slightly larger and you’ll get approval” and I was like, “OK, fine. ‘I honestly don’t give a s***.”

‘But then they showed me the first copy and these things must have been double-Es - and they were down to my knees.

‘And I was like, “I don’t mind you making them bigger, but don’t give me droopy breasts. They look like your grandmother’s t***’, the actress said.

She then went on to say the incident was certainly not the first time she has had her chest digitally enhanced.

She explained: ‘I did one magazine and found out you’re not actually allowed to be on a cover in the US without at least a C cup because it turns people off.

‘Apparently they have done market research and found that women want to see no less than a C cup on other women. Isn’t that crazy? ‘So they made my t*** bigger for that as well.’

Miss Knightley’s comments will be seen as refreshingly honest in an industry fettered by publicists and agents watching every word. the actress has an agent, but unusually, no publicist.

As she looked alluringly over her shoulder in a backless dress last night, Keira Knightley left no room for doubt she is now an established Hollywood star.

The 21-year-old arrived at the European premiere of Pirates Of The Caribbean: Dead Man’s Chest looking like she was the bounty.

In a bronze Gucci dress offset with a Cartier ring, bracelet and earrings - the jewels were said to be worth more than £100,000 - she appeared alongside her co-stars Johnny Depp and Orlando Bloom.

The sequel to Pirates Of The Caribbean: The Curse of the Black Pearl is set to make millions at the box office.

Both her leading men appeared at the premiere at the Odeon in Leicester Square to the delight of the excitable fans.

But Knightley, who made a name for herself playing a tomboy in the British film Bend It Like Beckham, was her usual down-to-earth self.

She excused her revealing dress saying: “I figured it was hot outside and if you can’t wear a dress like this on a day like today, then when can you?”

The first film took £350 million and a third Pirates of The Caribbean is already planned. Dead Man’s Chest opens nationwide on Thursday.