Hollywood star, enduring style icon and dedicated humanitarian – Audrey Hepburn remains recognised the world over as all three. But the Breakfast At Tiffany’s actress had a fourth role, perhaps even her greatest, as an advocate of female individuality in a conformist age of glamour.

“When Hollywood women were in flouncy dresses with big, curled and coiffed hair, she was in her black slacks and black jumper and flat shoes,” says journalist Wendy Holden, who has co-authored the first authorised biography of the star with Audrey’s eldest son Sean Hepburn Ferrer.

“She was a feminist icon ahead of her time. She made it okay for us to cut our hair short and not wear stilettos.”

Such was Audrey’s stardom both on and off screen that more than 30 years after her death, it’s hard to imagine there is anything new to learn about the woman who is the subject of 1,000 books, and counting.

But when Sean asked Wendy to help him reveal the real woman behind the image, flaws and all, he had his own revelation to tell – a deeply buried family secret unknown to the world.

It described the moment Sean, then aged 17, arrived at his Switzerland home and discovered Audrey had taken a near-fatal overdose following the traumatic breakdown of her second marriage to the Italian psychiatrist-neurologist Andrea Dotti, Sean’s stepfather.

“We both really wrestled with whether we should reveal such a painful and personal chapter in Audrey’s history, something that had been kept hidden for so long, which was a miracle given the Rome paparazzi followed her every move,” says Wendy, speaking ahead of the publication of Intimate Audrey.

“I knew it could potentially tarnish the reputation Sean has worked so hard since her death to protect, but he was adamant he wanted to tell her whole story and, if this was to be the definitive biography recording her whole life, it must be included, however difficult.”

Audrey’s 12-year marriage to Dotti was tumultuous due to his infidelities, which were often splashed across the newspapers to her humiliation. Wendy believes she stood by him, partly because she didn’t want Sean, the son of director Mel Ferrer, and his half-brother Luca, Dotti’s son, to be products of a broken home.

She recalls being shocked by Sean’s revelation. “He came home from school and found that Audrey’s maid Giovanna had found his mother unresponsive and just staring in bed.

“Audrey later told him that she hadn’t meant to end her own life, that she was just so tired of everything that had been going on and just wanted to go to sleep. Nevertheless she took an overdose of sleeping tablets and had to be transported to hospital to have her stomach pumped.

“Sean is very protective of Audrey’s legacy, he has devoted his life to protecting and maintaining it, but he felt this story needed to be told. So many women identify and worship Audrey and we both felt it was right in the end to show her vulnerability and pain, that she was human and authentic, not perfect as she was on screen.”

Before becoming an accomplished author, Wendy spent nearly two decades as a war and foreign correspondent.

“I think Sean wanted me to write the book with him because I have seen and experienced many of the things Audrey did when she was a UNICEF ambassador – and I have written about the Second World War and have an understanding of what she went through as a child in a Nazi occupied country, and also what she had witnessed in later life in refugee camps in war-torn countries,” she says.

She remembers growing up watching Audrey’s films on a Sunday afternoon and being thrilled by their lightness and luminous joy. The actress played a runaway princess in Roman Holiday, an eccentric socialite in Breakfast At Tiffany’s, a nun struggling with her faith in The Nun’s Story, a cockney flower seller in My Fair Lady and a bookstore clerk in Funny Face.

Yet it was only in the course of Wendy’s conversations with Sean, that she learned of “a sadness about Audrey that she carried with her from her childhood” beneath her animated performances.

The book details that painful childhood in occupied Holland during the Second World War and how Audrey, the daughter to an aristocratic Dutch mother and a British merchant, suffered from severe malnutrition alongside the 20,000 Dutch who died of starvation during the Nazi occupation.

The famine during the winter of 1944-1945, known as the Dutch Hunger Winter, saw Audrey barely surviving on hot water and staying in bed to conserve what little energy she had left.

As she said herself: “Being without food, fearful for one’s life, the bombings and all, made me appreciative of safety, of liberty, in the sense the bad experiences have become a positive in my life.”

Wendy believes it was the star’s formative experiences of war that shaped her into a humanitarian.

“Audrey was shocked to learn that, like many in the 1930s, her parents had been seduced by fascism and had even met Hitler [they posed for a photo with him]. I am sure she was mortified by her parents’ earlier association with fascism and she spent the rest of her life working for the opposite of that, of fighting oppression.”

Long after her film career ended Audrey would draw on her own experience of war time oppression to advocate fiercely for those living under hostile regimes in war-torn countries. The book reveals for the first time how she helped a Vietnamese family escape after they were put into forced labour during the Vietnamese War. The father had been a Vietnamese diplomat in Rome and his son Bang had been best friends with Sean.

When they were repatriated to the south and forced to live under communism Bang wrote three identical letters to Audrey, sending them to the UN, UNICEF and the American Embassy.

“By some miracle one of the letters reached her and she set about rescuing the whole family and getting them to France,” says Wendy. “There was no publicity, no fanfare, just Audrey remembering a little boy who had befriended her son all those years ago in Rome and wanting to help. She had no ego.”

Audrey’s childhood experiences also informed her approach to motherhood. “Her own father abandoned her as a child and her mother was hard and distant whereas Audrey was the exact opposite with her own children,” says Wendy.

“She would have stayed in her marriages if she could have for the sake of her boys. Being a good mother was everything to her and her closeness to Sean in particular was self-evident from my time with him.”

Sean himself writes of Audrey: “I came to realise my mother was a lioness. She was soft and strong at the same time and had the gentlest of hearts.”

Despite the extremely difficult and public break-up of both marriages, Wendy says Audrey did her utmost to ensure both sons had a good relationship with their fathers. Indeed both of her ex-husbands were at her house in Switzerland when she died on January 20, 1993, aged 63, from a rare form of abdominal cancer.

Sean was with her at the end and remembers their final conversation in the book. “She tells him she can see the children she couldn’t save in the refugee camps and that they are okay now,” he writes.

There are many touching personal memories like this that are sure to delight Hepburn fans young and old. They include the story of Audrey’s famous grace and poise.

The actress wanted to be a ballet dancer from an early age but the war meant she missed years of training. She moved to London after gaining a scholarship with the prestigious Rambert Ballet School, but was told she wasn’t strong enough to make it professionally, paving her way into films as a chorus girl.

Yet it transpires Audrey’s upright posture and elegance isn’t just to do with her dance training. “She never wanted anyone to know but she actually got a piece of shrapnel in her neck during the war and it affected the way she could move her head and neck,” says Wendy.

In the book, Sean reveals that so ubiquitous is his mother’s image in everything from books and magazines to novelty items, that he often played the game of “three minutes to find grandma” with his children when they were young to pass the time in public.

Wendy too now plays the game. “She may have died in over 20 years ago now, but her image is everywhere still,” she smiles. “Like Sean and his children, I find myself looking out for her everywhere I go.”

● Intimate Audrey: The Authorised Biography, by Sean Hepburn Ferrer and Wendy Holden (HarperNonFiction, £25) is out now; buy from wendyholden.com or all good bookshops