The Kate: This season's must-have fashion accessory (aka Middleton & Moss)

Kate Moss and Kate Middleton don't, frankly, have much in common. One is an international supermodel with a rock-star boyfriend and a penchant for cocaine. The other is a nice, middle-class girl who looks likely to become a future queen.

But the recent arrival of Prince William's girlfriend at high-street clothing chain Jigsaw has changed all that and could propel the store into a lucrative new dimension.

Ms Middleton, 25 on Tuesday, may only be an assistant accessories buyer, but the relentless coverage of her every move since starting work in November is highlighting a company that has never actively courted the limelight. The great British public is a sucker for a famous face. Is Ms Middleton about to do for Jigsaw what Moss has done for Topshop?

When it works well, celebrity endorsement is a force to be reckoned with.

Burberry credits the "Kate Moss effect" for a 23 per cent increase in sales in the first half of last year. The designer label described the "extraordinary" demand for its £750 Manor handbag after Moss was pictured carrying it in an advert. Her ability to shift stock is legendary: paparazzi pictures of the 32-year-old wearing a certain piece of clothing mean the item sells out within days.

"For the biggest issue of the year, the cover has, yet again, been given over to Kate Moss," wrote Alexandra Schulman, editor of Vogue, recently. "I don't say this because she's a friend of mine, but because as a model and a personality, Kate Moss sells."

Topshop is doubtless hoping this will also happen when Moss's collection hits its shop floor this spring. "Her involvement there is a huge PR coup," says Tamasin Doe, fashion director of InStyle magazine. "In as far as any person epitomises a brand, Kate Moss certainly does it for Topshop."

Whether Ms Middleton's presence four days a week at Jigsaw's head office in Kew, south-west London, will usher in the Jigsawisation of Britain remains a moot point, but she is a fan of her brand. She is often seen wearing items from the range, known for its safe, countryside colours and classic English designs.

"Kate Middleton won't become a style leader as Moss is, but I think she will become an inspiration for women who are looking to achieve an attractively classic look," says Ms Doe.

But Ms Middleton's real contribution to Jigsaw, which turned over £88m last year and says it is undergoing a "period of creative transformation", is likely be more subtle. "The Jigsaw-Kate Middleton relationship is a completely new spin on celebrity endorsement," Ms Doe says. "It's a more gentle association, a more social endorsement of the brand."

"They've been very smart in not proactively promoting the fact that Middleton works for them," says Tina Fotherby, managing director of the Yes Consultancy, a PR firm. "The incessant interest in stories about Kate Middleton means we're going to be reading more and more about Jigsaw as well."

It could, of course, all go horribly wrong. "Companies get awfully worried when the face gets so big it eclipses the brand, and there are several examples of companies who sign up a big name only to find that consumers remembered the face of the advert, not the product," warns Ms Fotherby.

For example, Pepsi last year dropped Britney Spears and Beyoncé Knowles from its international campaign, saying the two pop stars were just too big for the brand. The Hollywood actress Catherine Zeta-Jones was also dropped as the face of T-Mobile in November.

Some believe Moss is now at risk of becoming too associated with Topshop to the detriment of her other lucrative endorsement deals. But, as far as Ms Middleton is concerned, for the moment at least, she is safe at Jigsaw.

"It can't really hurt Kate to be at Jigsaw at all," says Ms Fotherby. "Middleton and Jigsaw are a match made in heaven and it's great for her to be seen to be working like any other person, because it promotes her down-to-earth image. The problem will come if her relationship with Prince William turns sour."

THE MOSS FILE

Name: Katherine Moss

Age: 32

The Pedigree: Croydon-born comprehensive school girl who was discovered aged 14 by Storm Model Management.

The Look: Iconic rock-chick waif. Mixing cheap chic and haute couture with ease, she is famous for her ability to start trends, including Ugg boots, waistcoats and, most recently, skinny jeans.

The Product: Own range for Topshop.

The Accessory: Pete Doherty, singer in rock band Babyshambles and general bad boy. Their on-off relationship has been going on for a year and a half.

She says: "I love Topshop."

THE MIDDLETON FILE

Name: Catherine Elizabeth Middleton

Age: Turns 25 this week

The Pedigree: Middle-England daughter of millionaire parents. Attended private boarding school before completing history of art degree at St Andrews University.

The Look: Kings Road demure. Classic and very, very British, she epitomises the modern Sloane Ranger look: expensive jeans, crisp white shirts, knee-length skirts, kitten heels and softly tailored jackets.

The Product: Working as assistant accessories buyer for Jigsaw.

The Accessory: Prince William. Heir to the throne, geography graduate and army officer, whom she met at university in 2001.

She says: "I'm not really fussed about fashion."