I will always remember the moment I was asked to be one of Princess Diana's bridesmaids. It was 1981. I was 13 and on holiday in the Bahamas. A call came through from His Royal Highness, the Prince of Wales, who is my godfather and second cousin. I was horrified. I was going to have to wear a dress.

I was a tomboy from rural Oxfordshire, never out of jodhpurs. I had been a bridesmaid before, for the Duke of Westminster, Lord and Lady Romsey, and my nanny, and all had entailed awful dresses.

But Charles was, and is, a remarkable godfather. I adored him and still do. My mother had been a bridesmaid for the future Queen Elizabeth II and Prince Philip in 1947. So despite the dress, I glowed with pride to have been asked.

But first, I had to practice. It was during these rehearsals that I got to know Diana, whom I first met at a dress fitting. She always seemed more like a head girl than a princess-in-waiting, with never a shy moment in private.

"For each fitting, I would be secretly taken out from boarding school and driven to London, where we were rushed past the world's photographers, lying in wait and ready to attack."

But I also saw glimpses of what hell it must have been to live besieged by the press. For each fitting, I would be secretly taken out from boarding school and driven to London, where we were rushed past the world's photographers, lying in wait and ready to attack.

The fittings took place inside the studio of designers David and Elizabeth Emanuel. Frill after frill, pin after pin, hour after hour, we stood silently as minions brought the Emanuels' creations to life. Although I found all those petticoats, puffed sleeves, and bows hard to forgive, it was an intimate and informal time with Diana.

Each bridesmaid had been chosen for personal reasons of the bride and groom. I knew most but was especially relieved to have the lovely Sarah Armstrong-Jones there (the queen's 17-year-old niece) to guide us gently at the rehearsals, which were orchestrated by Buckingham Palace and slightly intimidating.

Sarah and I were given the responsibility of Diana's 25-foot-long train. Twenty-five feet was unheard of, record breaking, and a bloody nightmare. Manipulating that much taffeta and antique lace in and out of the small state carriages posed considerable complications. We practiced with a long dust cloth tied to Diana's waist. She stood patiently as we were shown how to fold and unfold the fabric so it would glide effortlessly behind her.

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Meanwhile, the royal wedding frenzy grew. The government named the date, July 29, a public holiday. People prepared to celebrate with street parties and built bonfires to light on the day itself.

On the eve of the wedding, the guns of the King's Troop Royal Horse Artillery fired a salute in Hyde Park to mark the first marriage of a prince of Wales in 118 years, followed by an unprecedented fireworks display. About half a million people flocked to Hyde Park. My older sister, Edwina, and I joined Sarah Armstrong-Jones and the rest of the royal family to watch the fireworks. When we left, it became apparent that with the surging crowds, we were never going to make it home. Princess Margaret suggested that I spend the night with her in Kensington Palace, as we were practically on its doorstep. I will never forget her appearing in her nightie, offering me the use of her toothbrush.

"Sarah and I were given the responsibility of Diana's 25-foot-long train. Twenty-five feet was unheard of, record breaking, and a bloody nightmare."

Early the next morning, Sarah and I went to Clarence House, the London residence of the queen mother, to say good morning to a fresh-faced Diana dressed in old jeans with a diamond tiara on top of her head. Diana was amused to watch footage of herself on a teeny television screen that had been found in the nursery, and she would flick anyone who got in the way. During one commercial break, an ad for Cornetto ice cream came on, and she started singing. Soon we all joined in. "Just one Cornnneeeetttoooo" could be heard from the top floor.

At 10:20 A.M. on the dot, the bridesmaids were ushered downstairs by scarlet-tailcoated footmen, leaving Diana to finish her final touches. I remember the moment she appeared at the top of the stairs. Everyone fell silent. She was radiant and ready to become the most famous of princesses.

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The streets were lined with screaming crowds, which tried desperately to catch a glimpse of Diana and her dress as she rode past in the Glass Coach.

Sarah and I waited for Diana in the Chapel of St. Michael and St. George, one of the many small chapels within St. Paul's Cathedral. As the coach arrived, we went down the steps to help her out and arrange the train. She turned briefly to us both. "Do your best," she said and began to mount the steps. We knew what that meant: If we pulled too much, straightening the material, her tiara and veil would slip. But if we didn't pull enough, the effect of the train would be lost.

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At the top of the steps, Diana looked back, paused for a moment to let us finish, took the arm of her father, Earl Spencer, and walked through the doors of the cathedral. A hushed silence followed before the wedding march began. It took Diana, with her five bridesmaids and two pages in attendance, three and a half minutes to walk down the red-carpeted aisle before the crowned heads of Europe, foreign royals, a congregation of 3,500, and close to a billion people tuning in.

I sat munchkinlike on a small red velvet stool, close to the king of Tonga. I only really remember my buttercup-yellow satin shoes pinching; they were a size too small. The bride's nerves showed only once, when she mixed up the prince's first and middle names—calling him Philip Charles, rather than Charles Philip. And then it was over.

"I remember the moment she appeared at the top of the stairs. Everyone fell silent. She was radiant and ready to become the most famous of princesses."

I returned from the cathedral to the palace, with the bells ringing and crowds cheering, in Queen Alexandra's State Coach. Of course, at the time it seemed perfectly normal to be trotting down the Mall in a horse-drawn coach.

The newlyweds, meanwhile, took the open-topped State Landau to Buckingham Palace, where they emerged on the balcony to give the crowds the kiss they had been longing to see. Though I'd been privileged enough to be on that famous balcony several times before, nothing could have prepared me for that day. We watched in awe as the mounted police were forced to break rank by the swarming masses flooding through from the Mall to the palace gates, screaming joyously for Charles and Diana.

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Inside, Patrick Lichfield, a cousin of the queen's, took the wedding photographs—and wisely came armed with a whistle. During shot after shot, Prince Andrew insisted on making jokes. Finally, everyone collapsed in laughter and exhaustion.

At last it was time for the wedding breakfast. God knows why it's referred to as such, because in fact it is a lunch. As soon as the cake had been cut, Diana, her sisters, Sarah, the younger bridesmaids, and I disappeared upstairs to help her change into her pink going-away outfit, designed by David Sassoon of Bellville Sassoon. The mood was girly and giggly. Diana thanked us and gave us each a kiss and a present: a pretty gold-rimmed Halcyon Days china box. Inside were two of the silkworms that had spun the silk for her wedding dress.

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And then she was gone, whisked away across the palace courtyard, with a "Just Married" sign and old cans attached to the Landau by Andrew and Edward, raising smiles from the queen, the queen mother, and Princess Margaret, who hotfooted after the car, laughing and waving.

A few weeks later, a rose from Diana's bouquet arrived. It had been set in Perspex to act as a paperweight, with a note in her charming schoolgirl writing—all that remained of the historic day.