Yahoo! actively collaborated with the investigation of Shi Tao, 37, who was sentenced last April, Reporters sans Frontieres (RSF) said.

His case was one of the most publicised in a recent rash of actions against journalists and internet users intended to control the flow of information. Yahoo!, Google and Microsoft’s MSN are locked in competition for China’s huge internet market. All have come under fire for censoring news sites, search engines and web logs that displease the Government.

Shi was arrested last November at his home in the northwestern province of Shanxi. A court convicted him for posting on a foreign website a circular that the authorities had sent to his newspaper, the Contemporary Business News. This warned the media not to report on the 15th anniversary of the 1989 Tiananmen Square massacre because it could be exploited by dissidents.

According to RSF, the court heard that Yahoo!’s Hong Kong subsidiary had provided investigators with information that enabled them to trace Shi through his e-mail account.

The Paris-based group said: “We already knew that Yahoo! collaborates enthusiastically with the Chinese regime in questions of censorship, and now we know that it is a Chinese police informant as well.”

Yahoo!’s Hong Kong office declined to comment. RSF, which campaigns against the prosecution and detention of journalists, predicted that Yahoo! would defend itself by saying that it obeyed the laws of the countries in which its operates. “But does the fact that this corporation operates under Chinese law free it from all ethical considerations? How far will it go to please Beijing?”, it asked. “It is one thing to turn a blind eye and it is quite another to collaborate.”

In 2002 Yahoo! signed a “Public Pledge on SelfDiscipline for the China Internet Industry”, agreeing to abide by censorship regulations. Last month it agreed to buy 40 per cent of Alibaba.com, China’s leading e-commerce firm, for $1 billion (£550 million) the biggest foreign investment in the country’s internet sector.

Beijing has spent huge sums, much of it with foreign corporations, developing filters to block content and systems to track the use of the internet.

Users of the free blog service provided by Microsoft have found that they are unable to enter words such as “democracy”, “Taiwan independence” or “human rights”. These words come up with a series of asterisks.

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