Google Inc is working with U.S. and European officials to build a case that would argue Internet censorship acts as a trade barrier, a top company executive said on Friday.

Google shut down its mainland Chinese-language portal earlier this year over censorship concerns and a cyber attack it said it traced to China.

China has the world's largest number of Internet users and while the market has boomed, Beijing has kept a tight grip over sensitive content on subjects like politics and ethnic unrest.

Robert Boorstin, Google's director of corporate and policy communications, said the company is working with the Office of the U.S. Trade Representative, the State Department, Commerce Department and European officials to build a case to take to the World Trade Organization.

Such a case could help U.S. tech companies seeking greater access to Chinese consumers while furthering the U.S. government's human rights agenda.

U.S. Secretary of State Hillary Clinton and her technology advisers have promoted Internet freedom as a basic human right.

Google believes very strongly, as do other companies, that censorship is a trade barrier, Boorstin said during a panel discussion on Internet censorship hosted by the Media Access Project, a Washington-based public interest group.

Boorstin said Google wants to demonstrate that censorship results in fewer search pages, and that limits the capacity of the company to enjoy fair trade and the ability to operate on a level playing field with competitors such as China's Baidu Inc.

Our goal is to maximize free expression and access to information, he said. This is a very important piece of business for us.

The United States is studying whether it can legally challenge Chinese Internet restrictions that hurt Google and other U.S. companies operating in China, but the top U.S. trade official said in March that direct talks with Beijing might yield faster results.

A WTO case over Internet censorship would be the first of its kind and could take years to be resolved.

Last month a top European Commission official said China's Internet firewall was a trade barrier and needs to be tackled within the WTO framework.

Dutch-born Neelie Kroes, EC vice-president who is also in charge of Europe's digital agenda, said the firewall was a trade barrier as long as it blocked communication for Internet users, preventing the free flow of information.

Google's Boorstin said the company's attorneys and advisers are working on making the best case possible to present to the U.S. Trade Representative, but he warned it would not be an easy task and would not be resolved soon.

It's got to be pretty solid and it's not quite clear yet exactly how solid we can make it, he said.

I would not be looking for a conclusion in the near future, he said. These kinds of things take a long time.

(Reporting by John Poirier; editing by Tim Dobbyn)