Using a list of 'subversive' keywords including 'democracy', 'human rights' and '6-4' (the date of the Tiananmen Square massacre), Reporters Without Borders measured whether the results returned linked to sites authorised or not authorised by the Chinese authorities.
A massive 97 per cent of the results returned by Yahoo China were to authorised sites, making it more restrictive than Chinese competitor Baidu. Google China returned 83 per cent authorised sites, compared with 78 per cent for MSN China.
The same search on Google.com showed only 28 per cent of authorised sites in the results.
The study also found that if users entered certain search terms into Yahoo, such as '6-4' or 'Tibet independence', they would be temporarily blocked from using the search engine for an hour. Only Baidu used the same technique.
Reporters Without Borders urged companies operating in repressive countries not to censor search results.
"We are convinced that these companies can still access the Chinese market without betraying their ethical principles. They must, however, adopt a firm and clear position in relation to the Chinese authorities," the organisation said in a statement.
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