She's famously flat-chested - but just as well known as a buxom pirate girl - and now Keira Knightley has revealed the secrets of her seemingly versatile cleavage.

Miss Knightley, who has described her naturally small breasts as 'pecs', has confessed that her busty transformation is all down to make-up.

The slightly built 22-year-old Atonement actress shot to worldwide fame in the blockbuster Pirates Of The Caribbean - with breasts spilling over the tight corset she wore in the role of Elizabeth Swann.

Scroll down for more...

Growth Spurt: Slimline Keira in Gucci at a premiere (left) and an enhanced Kiera for Pirates of the Caribbean (right)

But it was all an illusion, made possible by the film-studio art of "bosom make-up". It is a skill from Hollywood's golden age that fell out of favour with the advent of silicone enhancement.

Miss Knightley said: "They painted my t**s on me for the films, which is extraordinary because it's kind of a dying art form - in the past, they used to have whole sections of the studios devoted to bosom make-up.

"And I loved it, completely loved it. Because it was the first time in my life I had big t**s, and I didn't even need surgery.'

She said the process of creating the cleavage took 45 minutes every morning before filming started.

A make-up artist would apply a slightly darker shade of base make-up between Miss Knightley's own breasts to create a shadow to increase the 'cleavage effect'.

The actress's assets were digitally enhanced in movie posters for her 2004 film King Arthur. In British versions she appeared in her natural state, but in posters for the US they had grown to a C-cup size, causing much debate among cinema-goers.

Miss Knightley said she was anxious not to be typecast in her Pirates role and took the risk of appearing in the 2005 film Domino as a model turned bounty-hunter.

In an interview in Easy Living magazine the actress, who is dating actor Rupert Friend, tells how she solved the dilemma of nudity required in Domino after being told by director Tony Scott that the role called for her naked bottom to be displayed on screen during a sex scene.

"I said, 'You can have my t**s, but you can't have my a**.' And he was going, 'I just want the a**; I don't want the t**s.'

"Then one day Tony called me in to his office and there were three very beautiful women standing there completely naked, and I couldn't quite take my eyes off bits of them.

"And he said, 'Which one do you want, then?'"

The actress was being asked to pick a derrière-double. She said: "I did pick a very beautiful bum."

Scroll down for more pictures...

Uplifting: Keira's chest was digitally boosted for the U.S. promotional posters of King Arthur

Chanel babe: Keira strips off for the Coco Mademoiselle ads

Screen siren: Keira Knightley is a vision in green in Atonement