While the wedding dress designers for Princess Kate when she married Prince William in 2011 (Sarah Burton for Alexander McQueen) and the Duchess of Sussex when she married Prince Harry in 2018 (Clare Waight Keller for Givenchy) were unconfirmed until closer to the nuptials, Elizabeth and David Emanuel were widely known to be behind Diana’s bridal ensemble — causing a frenzy at their studio.

“When Diana came in for a fitting, there would be hundreds of people outside waiting to spot her,” Elizabeth tells PEOPLE exclusively in this week’s issue.

In an effort to keep the royal wedding dress a secret until the big day, “We had to put shutters up on our windows, and we used to put bits of fabric, in different colors, in the bins just to throw the journalists off the scent,” Elizabeth says.

Their efforts worked, and Lady Diana Spencer’s wedding dress — and its statement 25-foot long train — became an instantly iconic style moment when she arrived at St. Paul’s Cathedral in London on July 29, 1981, to marry the future King Charles.

As excitement and pressure mounted before the wedding of the century, Diana found the studio to be an “oasis of peace,” Elizabeth says. “She would go upstairs and chat with all the seamstresses. She loved browsing through the rails because this was a new world for her.”

Unbeknownst to the 20-year-old bride and the millions watching on television, the Emanuels had crafted a second, mystery dress as a precaution for any unforeseen circumstances. “I was a bit neurotic, and I thought, ‘What happens if somebody breaks in and steals the dress or something spills or there’s a fire or it gets stolen?’ ” Elizabeth says. “So I thought, ‘I’m gonna make a backup dress.’ “

The designers didn’t tell Diana that there was another dress waiting in the wings and never showed it to the future princess.

“Neither of us wanted to worry her,” says Elizabeth. “It was a complete secret.”

The spare dress was inspired by a pink gown the Emanuels made for Diana to wear at a private ball a few days before her wedding and was never finished. The silk was white, “not the deep ivory that the royal wedding dress was made of,” Elizabeth says. The backup gown featured slim sleeves with frilly cuffs and ready-made fabric, unlike the intricate designs Elizabeth hand-embroidered on the dress Princess Diana wore.

“I just thought, ‘If anything happens we’ll finish it off and have it ready,’ ” says Elizabeth, adding that the extra dress “likely ended up on some sample rail. It probably got reused, torn up, thrown out, who knows?”

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Now, after searching old sketches and thumbnail images from an old documentary, the 70-year-old designer, based in London, has recreated what she calls “the dress that never was” for a unique purpose: to exhibit it at the virtual Princess Diana Museum.

 “We never got to see that dress on Diana and thought it would be lovely to envision it,” says Renae Plant, the museum’s director and curator, who acquired it for an undisclosed sum. “You cannot put a price tag on history.”