Diane von Furstenberg has a favourite piece of jewellery: a gilded necklace that spells out the word “Feminist” in swirling gold letters. “I love to see the fear in people’s eyes when you say that word,” says the 67-year-old designer with a warm laugh.

Forbes Magazine recently listed her as the 68th most powerful woman in the world. She was a princess (she divorced Prince Egon von Furstenberg in 1972) who became an entrepreneur, heading up a business that is now worth £300m, and then a philanthropist campaigning for female empowerment. DVF, as everyone calls her, knows what it takes to triumph.

The root of her own success stems from one frock. When she introduced the print jersey wrap dress, in 1974, it was an immediate hit. Inexpensive and universally flattering, it offered women something feminine yet work-appropriate at a time when millions of them were embarking on careers for the first time. Summing up its power as a garment, she says: “It’s the dress that you can get a guy in and his mother doesn’t mind. You can walk into a boardroom full of men and make a presentation, and you can look beautiful and sexy in a way that is not offensive.”

She recalls doing exactly that herself in the mid-1970s, when she asked the Sears board to give her $500,000 to do a home line. It was an unheard-of sum in those days. Then as now, she never let sexism get in the way of achieving her goals. In The Woman I Wanted To Be (one of two publications she has released), she describes how she ended one relationship with a man who wanted her to give up her business and become a trophy wife. She also remembers sitting on a plane two years after the wrap dress had launched, and with $100m in licences to her name, reading about herself in a newspaper, when the businessman next to her leered: “What’s a pretty girl like you doing reading The Wall Street Journal?” Success, she believes, is the best revenge.

For DVF, the wrap dress was always more than simply a flattering frock. “I didn’t realise how important it would be in my life,” she says in her exotic European drawl. “It paid for all my bills and my children’s education, my home and my independence. I knew the woman I wanted to be — free and in charge of my life. Fashion turned out to be my path to freedom, and as I gained my own confidence with the success of my wrap dress, I gave confidence to others via that little dress.” She remembers in the 1970s watching women in fitting rooms excitedly trying on her dresses and become more empowered before her eyes.

It’s still a symbol of freedom today. “It was the first dress Ingrid Betancourt bought after seven years of being held captive in the jungle in Colombia,” she says with pride. To celebrate 40 years of the wrap, she gathered a group of international women to model its latest incarnation. They all remind her of her younger self: “Of how I felt at the beginning of my adult life, at a moment when the world is full of endless possibilities.”

These days, she does far more than just give women confidence through wearable, flattering fashion. Her philanthropy, mentoring and annual DVF awards are all aimed at helping women to help themselves. “If you are established, or if you have, through your work, a voice, I think it is your duty, your role, your pleasure and your honour to give voice to those who have none.”

Claudia Croft / The Sunday Times / The Interview People