A Chinese reporter who was sentenced to prison in 2005 after Yahoo disclosed details of his email has been released, a writer's group has announced.

Shi Tao had been released on 23 August, 15 months before the end of his sentence, the Independent Chinese Pen Centre said in a statement. There was no indication why Shi was released early.

Shi was arrested in 2004 and sentenced to prison the following year on charges of disclosing state secrets. He had sent details of a government memo about restrictions on news coverage of the Tiananmen Square massacre anniversary to a human rights forum in the United States.

Human rights activists and US legislators criticised Yahoo for disclosing details of Shi's email to the Chinese government, leading to his conviction.

Yahoo, based in Sunnyvale, California, said it was obligated to comply with Chinese government demands for information. But at a US congressional hearing in November 2007, the company's chief executive, Jerry Yang, apologised to Shi's family.

Shortly after that Yahoo settled lawsuits brought by the families of Shi and Wang Xiaoning, a dissident who was sentenced to prison on subversion charges after the company's Hong Kong affiliate disclosed contents of his email account to Chinese authorities.

Also in 2007 the World Association of Newspapers gave Shi its Golden Pen of Freedom award.

Yahoo turned over control of its email and other services in China to a local partner, Alibaba Group, in 2005. The US company bought a 40% stake in Alibaba.

Yahoo closed its email service in China last month and recommended users switch to a service run by Alibaba.