Which swimming costume sells for pounds 120 at Harvey Nichols and which sells for pounds 21 as M&S? A judge might have to decide, says Tamsin Blanchard
Two years ago, Liza Bruce's turquoise and blue-dipped swimming costumes went on sale at Harrods, Harvey Nichols and exclusive little boutiques around London, Paris and New York. To date, Bruce, an American who designs swimwear and hi-tech stretch clothing, has sold a discreet number of her swimming costumes to women willing to pay for a bit of individuality. But since Marks & Spencer put an almost identical costume on sale this spring, it would appear that sales have soared. It has become one of those holiday essentials in the wardrobes of women from Southampton to Scarborough.

Bruce has not sold the design to Marks & Spencer. Nor has she been recruited by the company (which has used Paul Smith and Betty Jackson as consultants in the past) as a consultant on its swimwear range. "I did not design their costume and they have not asked my permission to use my design," she says. "If M&S had asked me to consult on the range, I wouldn't have said no. It would have been a challenge. This has nothing to do with snobbery."

Around the time the swimsuits went on sale at 80 M&S stores around the country, Bruce started getting letters and phone calls from customers and stockists who were angry at seeing what they thought was their pounds 120 swimsuit on sale at M&S for a mere pounds 21.

So Bruce, who has an annual turnover of around pounds 3m, has decided to take legal action against Marks & Spencer, whose turnover last year was in excess of pounds 6bn. It is not just the design of a swimsuit that is at stake but the theft of a creative idea. "It's the whole concept they've merchandised," she says. "A whole look - it's their summer story."

High-street rip-offs of designer clothing are nothing new - in some ways this "trickle-down effect" is what the fashion industry is all about. And few designers take action when their designs turn up on the rails of a chainstore: they do not believe it will get them anywhere, and have anyway moved on to the next season's designs.

Nor is it just the high street that appropriates the ideas of designers. Designers do it to each other all the time. Indeed, people have pointed to certain influences at work in Liza Bruce's own designs.

Marks & Spencer,instead of simply offering to settle out of court and pay Bruce what to it is pin money (her claim for damages is expected to be around pounds 500,000), is refusing to back down. Its statement reads: "We take these allegations very seriously and, after careful investigation, have found absolutely no substance to their claim. As far as we are aware, no one has a monopoly on this fabric or dip dye technique." Indeed, Marks & Spencer is in a delicate position. As sponsors of London Fashion Week's New Generation Show, the company has access to every British designer's collection as soon as it hits the catwalk. It is not in the store's interest to become known as a company that steals the ideas of designers.

Harvey Nichols, the Knightsbridge store that has stocked Bruce's clothes since her first collection is backing her claim. But Bruce fears that her designs have been devalued, and that in a cut-throat industry she may have future problems with orders from stores such as Harvey Nichols. Her next collection, which uses the same fabric, has been put on hold. She was also hoping to float her company on the Alternative Investment Market to raise capital (up to pounds 20m) and to open a shop. For her, the stakes are high.

"In this country where cheap versions of designer clothes are widely available, I find that it is the designer who is perceived as the rip- off artist. People think, why does that garment have to be pounds 100 when I can go and buy it for pounds 20?" she says.

The situation is complicated by the fact that, as Tanya Sarne, designer for British label Ghost, has found, it is impossible to patent a particular fabric, or colour, even if as Liza Bruce did, you have built your reputation on the exclusive use of a particular fabric. Sarne, who consults for a division of Coates, sympathises with Bruce, and is watching her case closely.

Bruce claims Marks & Spencer has "harrassed" her suppliers into selling them the bubble crepe Lycra fabric that she has had exclusive right to. And she has enlisted the services of the law firm Stephens Innocent, which won an out-of-court settlement for Antoni & Alison when the T-shirt designers sued Giorgio Armani for stealing one of their designs.

Stuart Lockyear of Stephens Innocent believes Liza Bruce has a very strong case. "The fabric, dyeing technique, cut and colours of their swimsuit are all the same as those on the M&S swimsuit," he says.

Bruce, who has no formal design training, began in business with swimwear after a trip to Mustique. Her first collection sold to Neiman Marcus, Harvey Nichols and Harrods.

Three years ago, she launched a full clothing range which is shown on the international fashion circuit twice a year in New York. Her swimwear, bodies and leggings have long been sought after for their heavy Lycra which holds the figure in at all the right places. There is no comparison between a pair of cotton jersey leggings from M&S and a pair by Bruce, which have such a high Lycra content that they hug the legs and thighs like a second skin. But despite what M&S might say, there is a fine line (if any) between the M&S swimsuit and the Liza Bruce one.

"It's really hard work to stay in the fashion business at all," says Bruce. "You have to come up with 20 ideas to have one really good one. And it's really hard to keep designing things when someone's going to come along and mug you for ideas. I have to do something about it."