A Chinese activist who was arrested nearly a decade ago after a politically sensitive email he sent was disclosed by Yahoo has been released from prison, a writers' group has reported.

Authorities sentenced Shi Tao, a journalist and poet in the central Chinese city Changsha, to a 10-year jail term in April 2005 for "illegally providing state secrets to foreign entities". It was later revealed that the US internet giant Yahoo had given the Chinese government access to Shi's email account, facilitating his arrest.

Shi, 45, was released in the north-western city Yinchuan on 23 August, 15 months before the end of his sentence, according the Independent Chinese Pen Centre, an affiliate of the nonprofit writer's group Pen International. He is currently living with his mother in Yinchuan, the group said in a statement on Saturday. The reasons for his early release are still unclear.

In 2004, Shi used his Yahoo email account to send details of a Chinese government directive ordering journalists to play down the 15th anniversary of the 1989 Tiananmen Square crackdown to a New-York-based human rights forum. Beijing prohibits open discussion of the crackdown, in which People's Liberation Army soldiers opened fire on unarmed pro-democracy protesters, killing hundreds.

The case raised a maelstrom of controversy about the conduct of US internet companies abroad. Activists and human rights groups rebuked Yahoo for its complicity in China's suppression of dissenting voices. Yahoo spokespeople responded that the company was simply following local laws.

US Congress launched an investigation into the case in 2007; at a hearing, Yahoo's co-founder Jerry Yang apologised to Shi's family. "But soon after, the company only took minor steps to make amends for their actions, and stopped short of taking any decisive measures that would send a clear message to Chinese authorities that censorship would not be tolerated," the Amnesty International said on its website.

Shi was originally imprisoned in Hunan province, where he was assigned to forced labour and kept in squalid conditions; he suffered from heart problems, an ulcer and a skin condition, according to Pen International. In 2010, he was transferred to Yinchuan, where his conditions improved considerably.

Shi won two major journalism awards after his detention: the Committee to Protect Journalists' international press freedom award in 2005, and the World Association of Newspapers' golden pen of freedom award in 2007.

Yahoo surrendered control over its main Chinese web products in 2005 after forming a strategic partnership with the Chinese e-commerce firm Alibaba. It shuttered its Chinese email service in August.