Princess Margaret's Iconic 21st Birthday Dress Just Went on Display: 'It Was One of Her Favorites'

Princess Margaret was a true fashion icon of her time.

Earlier this year, a vintage dress worn by Queen Elizabeth‘s stylish younger sister was the centerpiece of a new exhibition in London, celebrating the influential designer Christian Dior.

Christian Dior: Designer of Dreams at London’s V&A Museum featured over 200 haute couture dresses, among them a stunning dress worn by a young Margaret on her 21st birthday. Photographed by Cecil Beaton wearing the bespoke off-the shoulder design, it became an iconic image of the late royal.

“We tend to think of Instagram stars as something of now but she was just as big, if not bigger at the time,” resident milliner at Dior, Stephen Jones tells PEOPLE. “She was so striking, graceful and beautiful and unlike the Queen [then Princess Elizabeth], she really had that freedom.”

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As heir to the throne, her sister had to be loyal to British designers where as Margaret could be more daring and flamboyant with her sartorial choices.

The dress, on loan from the Museum of London, has been lovingly restored – but the delicate cream silk fabric still shows signs of age or at least a good time.

“I imagine she had a very fun time in that dress,” assistant curator Connie Karol Burks tells PEOPLE at a preview of the show on Wednesday. “It was one of her favorite dresses and she wore it more than once – there are photographs of her wearing it in Paris at an event later that year.”

Made from organza silk and embellished with golden straw (yes, straw!), embroidery, sequins and mother-of-pearl detailing, the dress was an adaption of the “Matinee Poetique” design from the French couturier’s Spring/Summer 1951 collection. With its buttoned bodice, tiny waist and full skirt, it epitomized the designer’s famous New Look silhouette, which launched in 1947 and was a controversial style post-World War II when fabric was still rationed in Britain.

“Christian Dior absolutely loved to dress Princess Margaret because she looked fabulous in his clothes,” says Burks, who has been working on the exhibition alongside head curator, Oriole Cullen for the last year. “She was curvy with a tiny waist [smaller than 19 inches, she predicts] and the way that she held herself, her confidence, that’s what makes an outfit.”

In April 1950, Dior staged a top-secret show for Margaret, her mother the Queen and the Duchess of Kent at the French Embassy.

“The models had to curtsy and then back out of the room,” says Burks. “In the end, the Queen said, ‘Don’t worry about that because I can’t see the back of the dress. Can you just turn around please?’ ”

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Attending a gala dinner to celebrate the launch of the exhibition on Tuesday night was Lady Sarah Chatto and David Armstrong-Jones, Margaret’s children with the late Antony Armstrong-Jones, otherwise known as the Earl of Snowdon.