As much as one-tenth of the Internet's sites may be blocked by Chinese authorities as part of efforts to prevent access to information on dissidents, Taiwan, Tibet and other subjects.
A study by Harvard University's Berkman Center for Internet and Society found more than 50,000 of some 200,000 websites tested were inaccessible from at least one point in China on at least one occasion.
Using a narrower definition of blocked sites, the study concluded that 18,931 websites were being kept out of view of Chinese Web users.
Ben Edelman, a co-author of the report, said it is likely that similar percentages apply to the Internet as a whole. 'I do think it's highly likely that on the order of one-tenth of the Internet is filtered by China, or recently has been,' he said.
The latest study was broader than prior surveys and gives perspective to scattered reports about websites that are filtered or blocked by China.
The blocked sites also included those of news organisations including ABC News and CBS News, the BBC, CNN, Time Magazine, US-based Public Broadcasting System, the Miami Herald, and the Philadelphia Inquirer, the Alta Vista search engine, and a number of US universities.
A large number of other sites with keywords such as 'freedom,' 'democracy' and 'HIV' were also blocked.
China, with an estimated 45 million Internet users, has been widely criticised in the west for blocking websites and for jailing people for viewing banned sites and using the Internet for anti-government activities.