A new exhibition at Kensington Palace highlights the key role in the fashion life of the Princess of Wales played by David Sassoon, the son of Iraqi Jewish immigrants, and founder of couturier Belville-Sassoon.

Chatting with the ever-charming Sassoon, 87 about the exhibition, an intriguing story emerges of how royal fashion history may have been changed by an unfortunate encounter between a shy, 19-year-old Lady Diana Spencer and Bellville-Sassoon’s formidable vendeuse.

Sassoon tells me he has handed over his entire Diana Archive to the new exhibition and has done so happily because “unlike the V&A which never sends anything back,” he expects it all to be returned, but if not “Kensington Palace was where Diana lived, so it will all be in her home.”

His archive comprises not just sketches for the 70 outfits made for Diana from her engagement onward — some of which have her notes and suggestions — but includes handwritten thank-you notes and letters to Sassoon from Diana and — in the early days — from her mother, Frances Shand-Kydd.

Sassoon’s treasure-trove of drawings and memorabilia form a major component of Royal Style in the Making, the exhibition sponsored by the Blavatnik family and royal jeweller Garrard. As well as featuring never-before-seen items from the archives of Bellville-Sassoon it also showcases items from the archives of other celebrated royal couturiers of the 20th century such as Norman Hartnell and Madame Handley-Seymour, a favoured couturier of Queen Mary.

The exhibition, which opened on Thursday, also affords visitors an in-the-flesh look at arguably the most famous wedding dress of the 20th century which was, of course, made by the Emanuel design duo, one half of which — Elizabeth Emanuel, née Weiner — was Jewish.

Housed in the newly-refurbished Orangery at Kensington Palace, it explores the relationship between fashion designers and their royal clients. Alongside glittering gowns and stylish tailoring created for three generations of royal women, there are some star items. These include a rare surviving toile for the gown made for Queen Elizabeth the Queen Mother for the 1937 coronation of King George VI, several of Hartnell’s gowns for state occasions and Diana’s wedding dress with its spectacular 25ft, sequin-encrusted train, its bodice overlaid with panels of antique lace and its bow-trimmed, ruffle-garnished puffed sleeves.

But as Sassoon reveals, an infelicitous encounter between a shy, 19-year-old Lady Diana Spencer and Bellville-Sassoon’s formidable vendeuse may have changed the course of fashion history.

To understand how events unfolded, it is necessary to understand the dynamics at play in high-end London fashion in the early 1980s. When Lady Diana commissioned her wedding gown from Elizabeth (née Weiner), and David Emanuel, she had experienced the Emanuel “wow” effect after one of their shapely, strapless black gowns had set the camera flashes popping during the earliest days of her romance with the Prince of Wales. Despite this success, however, the Emanuels were not the obvious choice for a Royal Wedding gown. The obvious choice would have been Bellville-Sassoon. Around the time of Diana’s engagement to Prince Charles, in February 1981, its Knightsbridge atelier was a cross between Debrett’s and Hello! As well as numerous celebrities, it was the go-to occasion-wear salon for the Sloane set and for younger royals. As a fully-fledged member of the Sloane set and about to become a royal, Lady Diana Spencer would have naturally gravitated to — and been guided towards — Bellville-Sassoon for her royal wedding gown.

Indeed, but for the atelier’s fearsome vendeuse, they may have been asked. As the softly-spoken Sassoon reveals to the JC, she was not only, in all likelihood, responsible for losing Bellville-Sassoon the commission to make the iconic wedding gown, but very nearly lost them Diana as a client altogether.

Sassoon explains how it happened: several months before the engagement announcement, “a shy young woman came into the atelier at 5pm.” In a scene vaguely reminiscent of Julia Roberts being rebuffed in a high-end Rodeo Drive boutique in Pretty Woman, the vendeuse — a Romanov princess by marriage — gave short shrift to a client with the temerity to arrive so late.

Failing to recognise Lady Diana Spencer and anxious to leave promptly at 5.30, the vendeuse suggested to the young would-be client that she “try Harrods” for the formal suit she was seeking.

Smiling at the memory, Sassoon notes that Lady Diana Spencer did, indeed, follow the vendeuse’s advice and purchased, at Harrods, the rather matronly royal-blue Cojana suit seen in all the engagement pictures.

“We were very disappointed not to receive the commission for the wedding dress,” states Sassoon. “And we heard Diana was very reluctant to come back to us, but her mother persuaded her to give us another try.”

From then onwards, Sassoon made more than 70 outfits for Diana, including her wedding-day “going away” outfit; her wedding trousseau, ten outfits for her honeymoon and the dress she wore for Prince William’s christening.

Making for Diana over such a long period, Sassoon also observed her style evolution: “When I first made dresses for her, she was 19 years old and liked to wear pretty and romantic dresses. But over the years, her taste changed and she became more glamorous and sophisticated.”

Based on the charming notes and letters which Sassoon has loaned to the exhibition, we can see not only that Diana appreciated Sassoon’s designs, but also that, as Matthew Storey, curator at Historic Royal Palaces explains, she was “good at reading and understanding a fashion sketch.

“David could sketch something for her and she would understand from that sketch what the finished outfit would look like. She was part of the process.”

As Storey points out, “some of the sketches have little notes she has written, expressing her preference. She had a clear idea of what she wanted.”

For his part, Sassoon describes Diana as “a joy; an absolute joy.” Despite not being commissioned to make her wedding gown, he was invited to her wedding and, he notes sadly, he attended her funeral.

Royal Style in the Making is at The Orangery, Kensington Palace until January 2 2022.

Tickets: Adult £25.30 / Concession £20.30 / Child £12.70. Free for Historic Royal Palaces members.

www.hrp.org.uk/Kensington-Palace/whats-on/royal-style-in-the-making