William and Kate are quietly leading the modernization of the British monarchy, a brand that has been reinventing itself since the first William sat on the throne in 1066. While the current Prince William, a flight lieutenant in the Royal Air Force, provides a steady presence, his beautiful wife, the former Katherine Middleton, inevitably attracts most of the attention.

Kate's conservative yet flattering and stylish clothes, and her penchant for high-street emporiums like Topshop as well as name designers, have turned her into a major ambassador for British fashion. Items disappear from store shelves within hours of her appearing in them as women flock to replicate her style, and the designers she wears enjoy an instant international boost.

The "Kate" blouse from the Whistles retail chain and Reiss apparel's "Nanette" dress, which Kate wore in the official engagement photos, both sold out immediately and were reissued.

Designers whose global fame the new Duchess of Cambridge has enhanced include London-based Issa, Erdem, and Sarah Burton for Alexander McQueen. Ms. Burton did Kate's wedding dress.

With the world hoping to hear news of a royal baby, and the Diamond Jubilee celebration of Queen Elizabeth's 60-year reign and the London Olympics next year, Will and Kate are poised to take the Windsor image to even greater heights.

They "have contemporized the brand in the simplest way possible -- by being attractive representatives who behave immaculately," said Peter York, a marketing guru and cultural commentator. "Kate looks like a pretty, well-groomed American film star, while William comes across as likable and unconflicted -- unlike his father," Mr. York added. "William is a cipher about whose personality we know nothing; it's how the queen plays it, and it's how it should be."

The newlyweds recently solidified their Cool Britannia status by announcing a move to Kensington Palace. They will live in the same house where the Queen's racy younger sister, Princess Margaret, partied with the Beatles, the Rolling Stones, Rudolf Nureyev and others during the swinging '60s in London.

Mr. York is convinced that the London connection is vital to Will and Kate's status as an international brand. "London has taken over as the place where the world's super-rich plutocrats live," he said. "The Windsors are at the top of the tree, and if you want to promote yourself socially in London, you have to meet them."

While he conceded that "this is a fantastic example of rebranding," Daniel Dumoulin, founder of brand consultancy Sundance London, has a romantic view of the Windsors' ascendancy.

"Will and Kate represent the popular culture of the moment and have everything we aspire to: youth, success, beauty and love," Mr. Dumoulin said. "They are the benchmark for the perfect couple, and they are bringing a happy ending to the tragic fairy tale of Diana."