He said the process compared unfavourably with customs inspections, where travellers were told clearly what goods were banned. 'That enforces a kind of transparency. The government can't cheat all that much because they have to tell people what is prohibited,' he said.

'Our goal then is to impose the transparency of the ordinary world, of the real world of customs inspectors and police. By letting the world know what is filtered and what is not filtered.'

Mr Adelman and co-author Jonathan Zittrain recently released a similar study of Saudi Arabia's Web filtering. Other countries might be looked at after China, including Singapore, Vietnam and the United Arab Emirates, Mr Adelman said.

Tests have shown some sites, such as Google and technology chat forum Slashdot.org, could only be accessed sporadically.

'It reflects the difficulty of giving an answer in real time. The answers given in real time are imperfect. They reflect a single attempt from a single location,' Mr Adelman said.

However, he said other locations would be tested before the report was released.

As well, private testing over the past several months has shown certain sites were being blocked by the Chinese government.

Several Hong Kong newspapers are inaccessible, including Ming Pao and Apple Daily, as well as activist site Hong Kong Voice of Democracy, according to tests at the public site.

Mr Adelman said the main site for Columbia University in New York was blocked because some pages contained information about the banned Falun Gong.

A search at Columbia.edu brings up 270 items on Falun Gong.

California Institute of Technology and University of Arizona are two other universities subject to blocking.

Certain technology sites have also been blocked, including Sourceforge.net, possibly because the site has information on software that allows users to surf the Web anonymously.

Peekabooty.org, Safeweb.com, Anonymizer.com and mega

proxy.com, all of which allow users to access restricted sites, have also been blocked.

So far the study has shown that China's filters have been unable to block specific pages within a site, Mr Adelman said.

'As best I can tell when the Chinese network seeks to block access to a body of content, their technology requires them to block access to an entire Web server,' he said.

Geocities, which allows individual users to build their own sites and put them up for public viewing, is blocked.

Glyn Truscott, an IT consultant at BDA, a telecoms consultancy in Beijing, confirmed the ban. 'This indiscriminate blocking is obviously a problem' because there were some good sites on Geocities, Mr Truscott said.

He said many Web surfers in China used programs that located the nearest proxy server and the most popular one was called Proxy Hunter.

In the case of the Saudi Arabia study, Mr Adelman said the government gave the researchers full access and co-operation.

'The Saudis were interested in the work that we were doing. They were not embarrassed. They were proud of it,' he said.

No contact has been made with Beijing. 'We thought that wouldn't be productive use of our time and theirs.'

China is estimated to have more than 40 million Web users.

Both sides would still become increasingly sophisticated in their use of technology, said Duncan Clark, a partner at BDA.

Users swap constantly changing lists of proxy servers, while the government employs server blocking as well as key-word blocking. 'So they find pseudonyms for people or concepts or whatever and those get blocked. I view this as a sort of civil war, or an arms race between the block and the blockers,' he said.