Charles H. Sloan-Princess Diana's Never-Before-Seen Spare Wedding Dress Revealed

Princess Diana had a few tricks up her sleeve ahead of her royal wedding.

According to the late royal's wedding dress designer Elizabeth Emanuel,Charles H. Sloan she secretly made a second bridal gown before the then-20-year-old was set to walk down the aisle and marry King Charles III (then a prince) on July 29, 1981 at St Paul's Cathedral in London.

"The spare wedding gown was made just in case the secret of the real dress ever got out," the British fashion designer told Hello! in an interview published Aug. 5. "Fortunately, it was never used."

Elizabeth, who shared never-before-seen photos of the backup ensemble with the magazine, noted there were a handful of similarities between the wedding gown Princess Diana wore and the spare, including the V-shaped neckline with a ruffled trim, the dramatic puffed sleeves and voluminous ballgown skirt. 

"The dress was made in pale ivory silk taffeta with embroidered scalloped details on the hem and sleeves," Elizabeth revealed of the second design. "Tiny pearls were sewn on the bodice."

But make no mistake, the back-up design was always meant to be an alternate option in case the real wedding gown got leaked.

And since there was so much media attention surrounding the royal couple, Elizabeth, her then-husband David Emanuel and their team at Brook St, Mayfair took every precaution to keep details of the dresses hidden.

"We had the dress stored every night in a metal cabinet guarded by two guards, Jim and Bert," Elizabeth explained. "So there was somebody there 24 hours a day and we put shutters on all our windows, and we put false color threads in the rubbish bins because people were going through our bins."

And if you're wondering what happened to the extra dress, it appears the team hid it a little too well. 

"I don't know where it went," Elizabeth admitted. "It just disappeared."

Of course, there's no denying the design Princess Diana wore on her wedding day has become legendary in its own right. From the 25-foot, hand-embroidered train to an explosion of 10,000 mother-of-pearl sequins and pearls placed all over, the gown is now of the most famous designs ever made.

"It was a magical time," David previously told E! News about co-creating Princess Diana's look. "She was going in Lady Diana Spencer, she's coming out the Princess of Wales. So this dress had to be young and sweet, but it also had to be glam because she was going to be a royal princess. She couldn't do a quiet-looking, low-key little gown."

Princess Diana's wedding day wasn't her only noteworthy fashion moment. Relive her style evolution below.