After consistently out-selling its competitors over the past three years, it was revealed that Primark was part of a growing trend for cheap clothes that has seen one pound in every four spent on fashion in the UK going to low-end retailers.

But Arthur Ryan, who has refused to give interviews for 30 years and has never been seen by some long-standing employees, may not be celebrating for long. Lawyers are circling.

A landmark ruling last month could herald the end to catwalk fashion at high street prices and lead to a series of law suits against the likes of Primark and Topshop, whose clothes often bear striking resemblance to designer fashions.

The UK judge ruled in favour of cosmetics company L'Oreal which claimed that the Belgium firm Bellure was copying its brand by producing "smell-alike" perfumes that were packaged to look like L'Oreal.

Ryan could be forced out of the shadows of the fashion industry if Primark faces a court case over its designs.

The company already had to defend itself against allegations of copying designs. Last year, Monsoon and H&M took Primark to court for alleged copying of their lines. Both cases were settled out of court;

Primark paid out £23,000 ( 34,000) in a settlement relating to a Monsoon top and dress. But with the new ruling, companies could sue for higher damages.

Thayne Forbes, L'Oreal's expert witness in the case as a marketing analyst, said the implications of the ruling in the fashion world are wide reaching: "Companies that are sailing a bit close to the wind will now have to be careful.

The dividing line as to what people might think is all right has shifted quite a lot. It will now be more difficult to copy [designs] legally. Clothing that bears specific attributes and similarities to other designs are now more of a problem than they used to be, particularly in the high street."

Mark Shillito, a trademark lawyer at Herbert Smith, agreed: "There will be those who try to use this case to broaden the principles into other areas."

The effects could be catastrophic for the fashion industry, which appears particularly susceptible. "That is exactly what [the big retailers] are about: getting their version of what they see on the catwalk to the shop floor as soon as possible.

"Companies have people advising them daily on how close to the line they can get without crossing it."

One industry source said:

"The ruling is groundbreaking in that it has gone beyond previous rulings. But fashion is by its nature ephemeral."

It takes little more than a week for a key look on the catwalk to be replicated for a third of the price and then hit the high street. Mass fashion shops have their clothes made in factories in Bangladesh, China and Vietnam where labour costs are cheap which enables the high street stores to sell fashion at bargain prices.

The company's remarkable success has not been at the hands of a major publicity drive, but has come despite its owner shunning the limelight. Primark sales topped £1bn last year. Cheap is suddenly chic.

Sarah Harris, Vogue features writer, says "it isn't just about the prices. These are often really, really good catwalk copies, possibly the best out there.

Take the naval jacket for example. The detailing is absolutely right . . . the buttons, piping, the ruching at the back."

According to Grazia magazine's Paula Read, even fashion editors can be found comparing notes on Primark's latest offerings. "I find fashion editors in Milan talking about Primark, which is kind of extraordinary."

Retailer looking to expand despite stagnant sales PRIMARK quashed fears that consumers were tiring of cheap fashion last week by announcing that it is still on the hunt for more UK space, despite adding 40% to the size of its estate over the past 12 months.

George Weston (right), chief executive of Associated British Foods, which owns the discount clothing chain, said Primark was looking for space in places such as Edinburgh and Milton Keynes where it was not represented.

Despite an abrupt slowdown in the chain's underlying sales this autumn, operating profits for the year grew 18% to £166m ( 250m) on revenues up by the same percentage to £1.2bn. It is adding a further 45% to its floor space, giving it 4.5m square feet by September 2007.