LONDON (AP) — It was black and strapless, with a sassy sequinned flounce at the bodice and a gloriously full, swishy skirt.

The dress was, Lady Diana Spencer thought, so grown-up, just right for her first official engagement after the announcement she was to marry Prince Charles. But when photographs emerged of the then 19-year-old Diana emerging from a limousine at a March, 1981 charity event — all creamy shoulders and ample decolletage — there was a minor scandal over the revealing cut.

According to Elizabeth Emanuel, who designed the ball gown with her husband David, they didn't realise the furore the dress would cause. "She just looked fantastic. At that time, she was curvy. Not fat in any way, but she had cleavage — we love cleavage. And she looked great in this dress," Emanuel said. "We in no way expected there to be such a reaction. And I think from that moment on, Diana became a fashion icon."

The dress had been missing for years until David Emanuel recently discovered it in a plastic bag at his home. Along with other garments worn by Diana and designed by the Emanuels, it is to be auctioned off June 8 in a sale that includes the silk chiffon blouse chosen for the Princess of Wales' official engagement portrait by Lord Snowdon, and the calico prototype used to fit her famous ivory wedding gown.

Prince Charles reportedly didn't like the ball gown she wore to the charity event, because he thought black was for people in mourning. Diana thought it was tres chic, and anyway, she had nothing else to wear. "She was unsophisticated at that point, and when I look at the wedding dress and the black ball gown I can see a young girl's dream of the ultimate party dress or romantic dress," said auctioneer Kerry Taylor, whose eponymous firm is handling the sale. "So there's an innocence about these early pieces.

"Later on she became very svelte, very sophisticated, very elegant," Taylor said. "But here we see just a very beautiful, innocent young girl, and the clothes reflect that."

For Britain's voracious press it would kick off a long love affair with the princess. "Up until that point, they'd seen her as a floppy-haired puppy," said Christopher Wilson, a seasoned observer who has written extensively on the royals. "And that's the moment Fleet Street fell in love with her. It all stretches back to that one picture."

Taylor and Elizabeth Emanuel are hopeful the collection will go to a museum — the black dress is expected to fetch between $44,000 to $73,000 and the prototype of the wedding dress between $12,000 to $17,000 — but understand that Diana's legacy means there's a strong chance a private collector may snap up the garments.