Diana's iconic wedding dress star of royal fashion exhibition

Princess Diana's wedding dress for her 1981 marriage to Prince Charles was one of the best-kept secrets in fashion history.

The gown sparked such intense interest that young designers David and Elizabeth Emanuel locked the ivory silk dress, which had a 25-foot (7.6-metre) long train, in a safe at night.

Plucked from obscurity for the commission of a lifetime, the pair even took to putting dummy bits of fabric in the studio's bins to throw anyone rummaging through them off the scent, according to an exhibition of royal fashion, including Diana's iconic dress, that opens on Thursday.

The exhibition -- Royal Style in the Making -- at the Orangery at Kensington Palace, Diana's home until her death in a car crash in Paris in 1997, focuses on the work of designers who dressed not just Diana but also Queen Elizabeth II, Princess Margaret and the Queen Mother.

Trimmed with vintage lace, pearls and thousands of sequins, the train of Diana's dress was the longest ever for a British royal bride and memorably appeared crumpled as she emerged from her carriage at St Paul's Cathedral.

Luckily, the designers were on hand to smooth it out.

"I think it goes to show that you can plan for everything, but on the day there'll always be something," the exhibition's curator Matthew Storey told reporters ahead of the opening.

"It's a very big dress. It was a very small carriage," he said.

In a video at the exhibition, Elizabeth Emanuel recalled Diana phoning to ask her and David to make the dress.

"It was one of those strange moments where you know your life is never going to be the same again," she said.

The exhibition, which runs until January 2, chronicles some of the hard toil behind the dress, featuring photographs of the seamstresses as well as the keys for the safe where it was safely deposited nightly.

The exhibition also highlights Diana's growing sense of personal style and evolution from girlish frills to sleeker, more impactful outfits.

With her wedding dress "she kind of left it to us really", Emanuel said.

But another designer she had a close relationship with, David Sassoon, lent the organisers archive documents that show her getting more involved.

She scribbled a comment on one drawing: "This in dark blue please" and in a handwritten letter asked for a dress pattern to be altered.

In another video, Sassoon recounted that Diana was "very shy" when they first met, but later became "very hands-on in selecting exactly what she wanted".

She "understood what the public wanted from the clothes she wore", he said, noting she "loved to break the rules", often not wearing gloves or a hat, as royal protocol required.

Her sons Princes William and Harry loaned both Diana's wedding and going-away dresses to the exhibition.

The creators said they did not know whether the pair would attend.

Diana would have turned 60 on July 1 and Harry and William are expected to unveil a long-awaited statue of her in a garden at Kensington Palace.

The exhibition comes as the princes have recently spoken more about their mother's pain at the end of her marriage and their sense of her legacy.

The popular drama series "The Crown" has also recreated some of her most famous outfits.

"I think her style is being celebrated again," Storey, the curator, told AFP.

"I think her promotion (of) and work for British fashion designers is a really important story."

The exhibition also explores the long-standing relationship between designer Norman Hartnell and the Queen Mother and Queen Elizabeth II.

The son of London pub owners, Hartnell began designing for the Queen Mother in the 1930s.

During World War II, she made a point of dressing up to visit bombed-out Londoners, Hartnell's biographer Michael Pick said in a video.

She would never wear sombre black or "unlucky" green, he said.

Hartnell later made Elizabeth's wedding and coronation dresses and the exhibition shows appreciative letters she sent him.

The most overtly sexy dress at the exhibition belonged to Princess Margaret and was made for a costume ball in 1964 by theatrical designer Oliver Messel.

With its low-cut, gold brocade-trimmed bodice, the dress was based on Georgian era fashion.

Princess Margaret was married to Messel's nephew, Antony Armstrong-Jones. After Messel's death in 1978, Princess Margaret stored his archive at Kensington Palace, showing their close relationship.