As many as one in 10 websites may be deliberately blocked to users in China, a US study suggests.

Sites containing sexually explicit content were among those blocked, but they also included sites on sensitive topics such as Tibet, Taiwan, and dissident activity, say the authors of the report, at Harvard University's Berkman Center.

Tibet isn't a subject China wants to discuss

They found that while only 13.4% of well-known sexually explicit sites were blocked, 100% of the top 10 sites produced by searches using keywords such as "Tibet", "Taiwan" and "equality" were blocked.

Attempts by Chinese authorities to restrict web access have been previously documented, but this study indicates the scope of the curbs.

"People often ask us and ask others, what is it that's blocked in China?" said one the authors, Ben Edelman. "For that kind of a person, we found that producing a list of blocked sites was helpful."

The authors compared access to the internet from the US and China. They requested 204,012 distinct websites, and say they found that more than 50,000 were inaccessible from at least one point in China - but not the US - on at least one occasion.

To try to ensure that glitches in service provision were not responsible, the authors then attempted to access blocked sites again from another point in China. They found that at 18,931 sites were unavailable from two points in China on separate days.

Restricted sites fell into some of the following categories:

• Dissident or democracy sites, such as Amnesty International, Human Rights Watch, and "dozens of Falun Gong and Falun Dafa sites"
• News sites: BBC News Online was consistently inaccessible, with only partial access to such sites as CNN and Time Magazine
• Health sites, including the Aids Healthcare Foundation and Internet Mental Health
• Taiwanese and Tibetan sites
• Religious sites, including the Atheist Network and the Catholic Civil Rights League

The authors suggest that methods of denying access to certain parts of the web from China have grown more sophisticated, with at least four filtering methods now in operation.

But although "there is some evidence that the [Chinese] government has attempted to prevent the spread of unwanted material by preventing the spread of the internet itself, [there is] a concomitant desire to capture the economic benefits of networked computing," the report says.

This has led to multiple strategies to control access, such as encouraging internet use in cyber cafes "rather than in private spaces so that customers' surfing can be physically monitored by others".