Google Inc. (GOOG) should continue to let advertisers use the trademarks of major brands as search keywords but should be liable for their wrongful use, according to a legal opinion prepared for Europe's highest court ahead of a final ruling on the issue.

Google is before the European Court of Justice, or ECJ, in connection with a French case in which a number of companies, including luxury goods giant LVMH Moet Hennessy Vuitton (MC.FR), complained that Google had infringed their trademark rights by allowing advertisers on the world's dominant search platform to use company trademarks as keywords. When a search is made on these keywords, the results will include links to rival products or to firms selling counterfeit goods in the sponsored links section. The advertising of counterfeit goods is against Google's terms of service.

The French court hearing the case, the Cour de Cassation, is asking the ECJ to rule on whether trademark owners can legally prevent Google from selling the right to use their trademarks as keywords.

"Google has not infringed trademark rights by allowing advertisers to buy keywords corresponding to registered trademarks," said the court's Advocate General Poiares Maduro.

However, Google may be liable for featuring content in its adwords program that directly violates trademarks, Maduro said.

The ECJ's advisor said that in most cases Google's use of trademarked keywords doesn't infringe the law, as Internet users rightfully expect more information and advertising to be returned as a result of a search query than just the products of the firms that own the trademarks.

"This opinion, if followed by the court, will be a major defeat for brand owners and would mark victory in Europe for Google's AdWords business,"says Adrian Heath-Saunders, a trademark partner at law firm Wedlake Bell.

Google is currently in litigation over its AdWords business in countries including England, France and Germany.

"We believe that selecting a keyword to trigger the display of an ad does not amount to trademark infringement, and that consumers benefit from seeing more relevant information rather than less," said Harjinder Obhi, Google's Senior Litigation Counsel, in an email.

However, the court advisor said that the search engine should still be considered liable for the wrongful use of keywords, with the burden of proof lying with the trademark owners.

Common infringements include using a trademark for selling counterfeit goods, as well as trying to confuse the consumer by using a trademark that is very similar to a known brand.

Google has argued that it is an online platform, simply offering a space for advertisers, and it should be advertisers that ensure they don't infringe trademarks, as it wants to avoid the huge expense of policing everything that is done on its sites.

Maduro said Google isn't simply an advertising platform as the search giant has a financial interest in users clicking on advertising links, for which it receives revenue based on the number of clicks made.

The opinion provides a clear answer for the first time in the E.U. as to how far trademark owners can restrict the use of their brands online. It is likely to provide guidance to other online companies such as eBay (EBAY), who have had to defend themselves in court over alleged counterfeit goods and trademark violations.

Google said it noted with interest the Advocate General's opinion, and awaits a final decision from the court in a few months.

An LVMH spokeswoman said the company acknowledges the opinion but noted that "it is not the court's final ruling." She added that the opinion had clearly pointed out that Google is responsible for what it sells, and can therefore be held responsible for selling key AdWords which reproduce brands like Louis Vuitton.

Legal opinions are routinely prepared in ECJ cases by senior court advisors known as Advocates General. The Advocate General's proposed ruling isn't binding, but final rulings follow the opinion in around 80% of cases.

-By Mike Gordon and Peppi Kiviniemi, Dow Jones Newswires; +352 691 180 766; mgordon.dowjones@gmail.com

(Mimosa Spencer in Paris contributed to this article)