According to press freedom organisation Reporters Without Borders, search engine firm Yahoo censors more sites than other search engines operating in China.
The group alleged that Yahoo China censored more results than Google China, MSN China and local search engine Baidu.
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 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.