Diana and Kate Middleton's Racy Fashion Choices

ABC News speaks to designer Charlotte Todd.

LONDON, March 7, 2011— -- Kate Middleton's first foray into fashion took place nine years ago at a St. Andrews University runway show.

She took to the catwalk in a sexy, see-through number made by former design student Charlotte Todd.

Indeed, the dress modeled by the princess-to-be wasn't meant to be a dress at all but was originally designed as a skirt.

And, as for being see-through: Well, that wasn't quite the plan either.

"It was meant to be worn with say a satin or silk slip underneath," Todd told ABC News. "It wasn't the intention to show the underwear."

So perhaps Middleton isn't quite as conservative as she is made out to be.

Todd, who has not been invited the royal wedding, made the dress out of gossamer-fine black and gold silk threads. The skirt-turned-dress was originally made for a design school project called "the art of seduction."

But, two years later, it may have helped seduce a prince. Prince William was in the front row for the fundraiser fashion show.

"He was sitting with his friend Fergus at the time," Katie Nicholl, royal expert, told ABC News. "He turned to Fergus and he said, wow, Kate's a knockout, she's gorgeous.

"I think that was really the turning point for them."

Said Todd: "Whether Kate knew what she was doing on that night, we'll never know. But I think she did. And I hope that she chose my dress to bag her prince."

After the show, the dress was packed away by its designer and kept in a box buried at the bottom of her parents' closet.

But now it has resurfaced and is going up for auction, alongside two dresses worn by Princess Diana.

"It was literally a bit of a show-stopper, this one," auctioneer Kerry Taylor said of the diaphanous dress. "It's interesting to think that our future queen once wore this."

$45 Dress Now Worth Thousands

Princess Diana herself had a racy and revealing coming out dress.

"The dress she wore the first time she was seen in public with Prince Charles after the engagement was announced in 1981 caused a storm," Taylor told ABC News. "It was black taffeta by Emanuel. It was cut so low."

It's probably safe to say that Britain's future queen won't be dressed in such an outfit again.

As for Todd, the designer, she swapped such delicate designs for a job as a retail manager at an aquarium in Bristol.

"It's not the best thing I've ever made," Todd said of the famous frock. "But it's certainly become the most iconic now, and it makes me very proud."

And if you're the lucky bidder, this see-through dress, which cost about $45 to make, could be yours for around $16,000 on St. Patrick's Day.

As for the real wedding dress, speculation abounded this weekend after the Sunday Times reported that the designer could be Sarah Burton, the creative director for Alexander McQueen.

McQueen is officially denying it, as is Clarence House, the royal brothers' official residence.

"The palace are going to be very noncommittal about this," Nicholl told ABC News. "They made it clear right from the start that nothing would come out on the dress. That Kate wanted this to be completely secret."

If true, though, the choice would be telling.

"This is an unusual choice, very cutting edge," Nicholl said. "Sarah Burton is dynamic, she's known for her very unique designs.

"Sarah and Kate are very close," Nicholl added, before going on to say how much William and Middleton enjoy their privacy.

"They are the most clandestine couple," the royal watcher said. "The engagement; everything was shrouded in secrecy."

"You just wonder if perhaps Kate has a contingency plan, something to fall back on, that there may be another dress once this comes out."