Tashi Phuntsog, who was released earlier this month, worked with a prominent lama, Tenzin Deleg Rinpoche, whom China has branded a "terrorist".

Human Rights Watch said Tashi Phuntsog needed immediate medical attention after being "physically broken".

The group called on China to explain the monk's deterioration in custody.

"Tashi Phuntsog entered prison as a healthy man in his early 40s, and he was cast out literally as a broken man," it said.

Tashi Phuntsog was detained in 2002, 10 days after Tenzin Deleg Rinpoche.

We welcome Tashi Phuntsog's early release, but Chinese officials have to explain how they allowed this to happen to someone in their custody Human Rights Watch

Tenzin Deleg Rinpoche was later tried and sentenced to death for carrying out bomb attacks in Ganzi and Chengdu, in south-west China, although human rights groups dismissed the charges as groundless.

China subsequently suspended the death penalty, and on Wednesday state media reported that his sentence had been commuted to life imprisonment.

The Chinese authorities have never said what Tashi Phuntsog was sentenced for, or why he was released early.

Before his release, he served nearly three years of a seven-year sentence, and was held in Kangding prison, near Chengdu.

Human Rights Watch said that as well as working with Tenzin Deleg Rinpoche, he had also been instrumental in stopping a government deforestation programme.

The area where the bomb attacks allegedly took place has a large ethnic Tibetan population and a history of independence activity.