China has the world's largest population of Internet users at more than 298 million. The communist government has the world's most extensive Web monitoring and filtering system, and it regularly blocks access to foreign Web sites.

While the government claims the main targets are pornography, online gambling, and other sites deemed harmful to society, critics say that often acts as cover for detecting and blocking sensitive political content.

Authorities launched a crackdown this year that led to the closing of more than 1,900 porn-related Web sites.

Google has struggled to expand in China, where it says it has about 30 per cent of the search market. The company launched Google.cn with a Chinese partner after seeing its market share erode as government filters slowed access to its US service.