Shoe designer Christian Louboutin may be forced to step back from its attempt to stop other companies from selling shoes with red soles — a design feature that the company claims it has exclusive rights to.

The French designer went to court in The Netherlands to prevent high-end Dutch chain Van Haren from shoes that look very similar to Louboutins, which are a mainstay in fashionable circles around the world. Known for their distinctive red soles, it is not uncommon for them to retail for more than $1,000 US a pair.

But an advocate for the European Court of Justice said in an opinion Wednesday that the company's attempts to trademark their design in the Benelux countries in 2010 and again in 2013 don't hold up to legal scrutiny.

In 2012, Van Haren started selling a high-heeled shoe with red on the underside, a move which prompted the legal fight. A Dutch court initially sided with Louboutin before appealing to the ECJ for its interpretation. The ECJ, in turn, sought legal opinions of its own before ruling.

In a press release, the court's advocate general Maciej Szpunar "expresses doubts as to whether the colour red can perform the essential function of a trademark."

That's because the colour of the sole cannot be considered to be separate from the shape of the sole, Szpunar said, and shapes are usually not protected under EU trademark law.

"A trademark combining colour and shape may be refused or declared invalid on the grounds set out under EU trademark law," the ECJ said in a statement.

The opinion is not binding, but the ECJ tends to follow the opinions of its advocates in its rulings. Which means the attempt to patent a shoe colour and design may soon be kicked out, and open the door to more knock-offs.

Szpunar said his analysis focused exclusively on the issue of the shape of soles and not on what the iconic red sole might be worth to the brand — he took "no account of (the) attractiveness of the goods flowing from the reputation of the mark or its proprietor".

"Whilst relevant consumers may instantly recognize a red sole shoe being uniquely associated with Louboutin, trying to persuade the courts to grant monopoly rights with such a 'badge of origin' may well be an insurmountable hurdle," said Sanjay Kapur, partner at intellectual property firm Potter Clarkson LLP.

Kapur said that if the ECJ were to follow Szpunar's opinion "then this could mean that Louboutin would not be able to stop its competitors, including haute couture fashion houses, from offering shoes with red soles."

Once the ECJ reaches its verdict, it will be up to the Dutch court that referred the case to take the final decision on whether Louboutin's red sole can be a trademark.

With files from Reuters News Agency