The revealing outfit caused some onlookers to wonder if the 21-year-old British starlet had bowed to Hollywood pressure to be super-skinny.

But 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."

She joked about her dress: "I haven't slashed anyone yet!

"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 European premiere of Pirates Of The Caribbean: Dead Man's Chest was held in London's Leicester Square yesterday.

Knightley was joined on the red carpet by co-stars Johnny Depp and Orlando Bloom.

Depp brought with him a cream Fedora and shades to protect him from the sun on the hottest day of the year so far.

Police had appealed to the public not to pack into the square amid fears they might wilt in the heat.

But a crowd of thousands ignored the warnings and were rewarded by an hour-and-a-half walkabout by the film's three stars.

Security men sprayed fans with water to keep them cool.

Knightley said: "It's quite crazy out there. It's really hot and I feel so sorry for all those people who have been standing out there all day. I know they're here for Johnny."

Other stars at the premiere included singer Charlotte Church, dressed for the weather in a white sundress and flip flops, and actress Mischa Barton in a blue babydoll dress.

Kimberley Stewart, tennis star Greg Rusedski and Bob Geldof's daughters Peaches and Fifi were also there.

The £100 million film is the follow-up to the 2003 original.

Depp reprises his role as Captain Jack Sparrow, which he based in part on Rolling Stone Keith Richards.

Richards is in talks to appear in the third film, which is currently in production.

Dead Man's Chest is released nationwide on July 6.