Lower courts in Belgium and Luxembourg asked the EU's top court for an opinion on a case brought by French shoemaker Christian Louboutin.

The firm argued Amazon itself was breaching its trademark by hosting adverts for shoes from third-party sellers featuring its trademark red sole.

The ECJ agreed, saying Amazon users would not necessarily be able to tell if a product was being offered by Amazon or a third-party seller.

"Amazon itself makes use of the sign registered by Louboutin where the user of its site has the impression that it is Amazon which is marketing, in its name and on its behalf, shoes of the trademark," said an ECJ press statement released in French.

The legal opinion does not decide the Louboutin case, which will revert to the courts in Luxembourg and Belgium.

But the opinion is binding on national courts in the EU from now on.

"This is a victory for all the brands that defend their expertise," said Louboutin chief executive Alexis Mourot.

Amazon said it was still studying the opinion.

The French shoemaker is famous for its red-soled footwear and claimed sellers on Amazon were offering "identical" products without the firm's consent.

Louboutin argued that "the disputed advertisements formed an integral part of Amazon's commercial communication", according to the ECJ press statement.

The court agreed, distinguishing Amazon from sites such as eBay, which operates purely as a marketplace for other sellers and does not offer its own products.

Amazon, the court pointed out, displays its own adverts alongside those of third-party sellers and offers the sellers marketing, storage and shipping services.

As a result, users could have the impression that Amazon was offering the disputed products.

Amazon -- along with fellow US tech giants like Apple, Google and Meta -- has faced a slew of legal cases across the EU.

On Tuesday, Amazon settled two EU antitrust inquiries concerning its treatment of third-party sellers.