“But who is making her dress?”

That is the most pressing question in fashion right now: the identity of the designer of Kate Middleton’s wedding gown for her marriage to Prince William of England on April 29 — known in Britain as “K-Day.” More than 2 billion people around the world are expected to watch the wedding on television or via the Internet, and what she chooses will have an impact on not only British fashion but British pride as well.

British bookmakers have been setting odds on the choice. “It’s all anyone talks about in London, and nothing but,” says Ed Burstell, managing director of Liberty, the legendary department store in London. “We’re going to get together and have a pool in the office.”

Every day there is a new rumor. In January, the British press said the designer would be Bruce Oldfield, who was a favorite of Princess Diana’s and made Queen Rania of Jordan’s wedding dress. In late February, fashion writers declared that it was Sarah Burton, Alexander McQueen’s longtime assistant who took over designing for the brand after McQueen’s suicide last year.

“If she went with Sarah Burton, that would be kind of amazing — really a statement,” Burstell says. “Having worked all that time with McQueen, Burton is an expert on the drop-dead gown. She could put a stamp on Middleton’s style.”

Last month, London’s royal wedding press corps — yes, there are scores of journalists assigned solely to this beat — reported it would be Daniella Helayel, designer for the British label Issa, who made that smart sapphire silk jersey number Middleton wore for her engagement announcement last fall. At another point, British designers Erdum and Alice Temperley were rumored to have been tapped. Last week, the name of longtime British designer Jasper Conran — a 1980s star and son of interiors man Sir Terence Conran of the Conran Shop — popped up.

And this week, a dark horse emerged: little-known Sophie Crans­ton, who has a small company called Libelula and made headlines in January by dressing Middleton in a figure-flattering black velvet coat over a sheer black top for a friend’s wedding in Yorkshire. Fleet Street went bananas over the outfit, declaring that Middleton shockingly went braless and denouncing the color choice. Remember: William’s ancestor Queen Victoria wore mourning black every day for her remaining 40 years after the death of her husband, Prince Albert. “Hope you won’t wear black on April 29, Kate,” the Daily Mail sniffed.

Royal family officials insist that no one other than Middleton’s bridal party and her designer will know who made the dress until she steps out of the Rolls-Royce in front of Westminster Abbey on her wedding morning. “The aim is to protect Prince William from knowing anything about the dress — something all brides do on their wedding day,” Prince Charles’s press secretary, Patrick Harrison, told reporters this month.

But this week, the Daily Mail reported that Cranston has finished the dress, that it is ivory satin and lace with a 10-foot train, that it is based on Middleton’s ideas, which were inspired by the gown Lady Diana Spencer wore at her wedding to Prince Charles, and that it is under lock and key at Clarence House, Charles’s residence.

All this secrecy is a departure from when William’s mother, Diana, married his father, Prince Charles, in 1981. Not long after Diana made headlines wearing a smart black dress designed by British designers David and Elizabeth Emanuel to her first public event with Charles, it was announced that the pair would be designing her wedding gown, too.

“Diana herself telephoned us and asked us, ‘Would you do me the honor of making my wedding dress?’ It was a life-changing moment,” Elizabeth Emanuel recalled last week from her studio in London. “We had always hoped and dreamed we’d get the commission, but we were young and just out of college, and there were other experienced designers around.” She points out that keeping the design a secret wasn’t as difficult then as it is today. “In 1981, we didn’t have computers and certainly not mobile telephones,” she said. “Today, there would be so much pressure on the designer, with people trying to get photos any way possible. Can you imagine?”

Princesses’ wedding dresses have been a public obsession since Princess Victoria of England married her first cousin, Prince Albert of Saxe-Coburg and Gotha, in 1840. Until then, brides traditionally wore color, and white was for mourning. Breaking convention, Victoria walked down the aisle of the Chapel Royal of St. James’s Palace in London dressed in a white satin gown trimmed with white lace, matching her veil and crown of orange blossoms. Soon, upper-class brides were wearing virginal white and lace.

Throughout World War II, brides opted for demure suits, often with a pretty corsage. But big white taffeta gowns came roaring back in the 1950s, the styles often set by celebrities and princesses-to-be. In 1950, MGM wardrobe designer Helen Rose made Elizabeth Taylor’s white satin wedding dress with seed pearls and a sweetheart neckline for her first marriage, to Conrad Hilton, and it was quickly knocked off and worn by brides around the world.

Rose also made the dress that Grace Kelly wore to marry Prince Rainier of Monaco in 1956. That dress — a high-neckline, long-sleeve ball gown of silk taffeta and antique Valenciennes rose-point lace — was one of the most copied ever.

For Diana’s wedding to Charles, the Emanuels created, per Diana’s specifications, an antique white silk taffeta fairy-tale confection with loads of ruffles, huge puffed sleeves, an enormous full skirt, 10,000 hand-embroidered pearls and sequins, and a 25-foot train. “She decided it was a fairy-tale wedding, and so we had to create a fantasy wedding dress,” Emanuel recalls. “And that’s what’s we aimed for. It was a transformation dress, from a kindergarten teacher into a princess, with the most amazing wedding ever, at St. Paul’s Cathedral and with carriages. We wanted to do a dress that would match up to not only Diana’s expectations but also the 750 million people who were watching.”

Though the dress was never reproduced by the Emanuels, it was knocked off immediately. “As she walked down the aisle, people were sketching it,” Emanuel remembers. “The next day, there were copies for sale all over London.”

The gown launched the Emanuels’ career — they later wrote a book about it called “A Dress for Diana,” though they are now divorced — and gave a much-needed boost to the British fashion industry. “Suddenly,” Emanuel says, “British designers were up there on the world stage.”

Middleton hopes to do the same. The Issa dress she wore for her engagement announcement sold out within hours. “The phone was ringing off the hook,” says Liberty’s Burstell. London-based online retailer Net-a-Porter carries it for $535, and it sells out immediately after each shipment arrives. “There’s no keeping it in stock,” a Net-a-Porter spokeswoman says. British supermarket chain Tesco has produced a short-sleeved version that sells for $25.

Although secrecy surrounds the wedding gown, bits of Middleton’s other fashion choices have leaked. A spokesman for Aristoc, a British legwear company, says Middleton has selected stockings from its bridal range. The British press believes that Victoria Beckham will design Middleton’s outfit for when the couple board their plane for their honeymoon, thought to be in Jordan. Last week, Middleton was spotted in the VIP salon of London’s Selfridges department store with her mother, Carole, and her sister and maid of honor, Pippa, trying on dozens of stilettos by shoe brand Gina.

Middleton’s longtime hairstylist, James Pryce of Chelsea’s Richard Ward Salon, will do her do — upon which she is expected to wear a Windsor tiara of her choice — and Ward will tend to Carole and Pippa. Fleet Street also reports that Middleton will wear diamond and pearl earrings, though it is not known whether they will come from the Windsor vault.

Once married, Middleton is expected to continue supporting British fashion, as Michelle Obama has with young American talent. But unlike Obama, “Kate will be a princess and inhabit the conservative world of the palaces, the horse races at Ascot, polo matches and a lot more pomp and circumstance,” says Robb Young, author of “Power Dressing: First Ladies, Women Politicians & Fashion,” published in February by Merrell. “Kate has to be very careful not to appear haughty or out of touch.” Young suggests she follow the leads of Princess Le­tizia of Spain and Queen Rania, who have a modern, informal style. “As far as royal style icons go, these two are probably the closest big shots for Kate to take a few cues from,” he says.

Middleton is also going to follow the lead of Victoria and break a bit of wedding convention: Sometime after the ceremony, she will auction her dress and donate the proceeds to charities, including her future father-in-law’s Prince’s Trust. “I think Kate Middleton understands the power of her choices,” Burstell says, “and she is going to be supportive of and have a lot of initiatives around British fashion.”

Thomas is a freelance writer.