A respected Chinese dissident has warned that the capitulation of Western internet companies to China's authorities is a more serious threat to free speech in the country than the Chinese government's filtering of what its citizens can access on the internet.

Speaking in Tokyo on Monday, Wei Jingsheng singled out Yahoo for its part in revealing information that helped land a journalist in jail in 2004.

"Let me specify Yahoo," he said at the Foreign Correspondents' Club of Japan. "They will track down internet users and help to sentence them. This really threatens the safety of internet writers in China."

Yahoo has been the target of criticism since just over a year ago, when it admitted providing authorities with evidence that helped land a local journalist a ten-year jail sentence.

The journalist, Shi Tao, was an editorial department head at the Contemporary Business News in China's Hunan Province. He was arrested in 2004 after sending an email to a New York-based website advocating democracy in China.

The email contained information regarding a Chinese government warning to officials that urged them to be vigilant ahead of the fifteenth anniversary of the Tiananmen Square massacre and to watch out for dissident activity. It was posted on the site under the alias 198964, the date Beijing crushed the student-led democracy movement - 4 June 1989.

With the information provided by Yahoo, Shi was arrested and convicted of divulging state secrets by a provincial court in April 2004, according to information from several sources, including Reporters Without Borders.

In its defence, Yahoo said at the time that it was just following local laws in handing over the information.

Wei, who spent more than a decade in prison in China for penning an essay in 1978 against the communist system, said that Yahoo's lobbying in Washington has helped prevent the company from being brought to account for its actions.

"What's regrettable is that some in the US Congress and government are speaking out for the Chinese government, so the Yahoo problem - the judicial problem - hasn't been resolved," he said. "As a big company, Yahoo has a lot of money for lobbying. I think this is a real problem. Everyone says that America's democratic system has been influenced by money and that's the problem."

Yahoo is not the only company that has come in for criticism for its actions in China. Google and Microsoft were named alongside Yahoo in an Amnesty International report published in July this year as companies that have "in one way or another, facilitated or colluded in the practice of censorship in China".

Microsoft was criticised for shutting down a blog on its MSN Spaces website following a Chinese government request and Google for offering a censored version of its search engine for China.