BRUSSELS, Belgium — Christian Louboutin's bid for exclusive rights to put red soles on high-heeled shoes ran into trouble on Tuesday when a court adviser said he may not be entitled to stop others from selling the same kind of sole.

The French designer went to court in The Netherlands to prevent the Dutch high street chain Van Haren from selling its own versions of his distinctive footwear, a favourite on celebrity red carpets.

But an advocate general at the European Court of Justice advised the court that Louboutin's trademark protection on the shade of red used on his soles, dating from 2010 and 2013, might be invalid. The judges usually, though not always, follow such advice.

"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, outlining Maciej Szpunar's advice to the judges hearing the case, which was referred by a Dutch court.

The red colour could not be considered apart from the shape of the sole, Szpunar advised, and shapes are usually not protected under EU trade mark law.

Szpunar "expresses doubts as to whether the colour red can perform the essential function of a trade mark, that of identifying its proprietor, when that colour is used out of context, that is to say, separately from the shape of a sole," the ECJ said.

Szpunar said his analysis focused exclusively on the issue of the shape of soles and not on the value of the branding of the shoes - 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 recognise 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.

Editors: Alastair Macdonald, Larry King and Adrian Croft.