Christians Against Torture says it is a "disgrace" that Shaker Aamer is still being held in Guantanamo Bay.
The Welsh group is urging Foreign Secretary William Hague to make a fresh public plea for the return of Aamer, a citizen of Saudi Arabia and the last British resident detained in Guantanamo Bay.
Aamer was cleared for release six years ago but remains behind bars. He has been held at the US base for more than 11 years and is on hunger strike in protest against his detention.
The Reverend Roy Jenkins, chair of Christians Against Torture, a Welsh churches' campaign, said Aamer had been "repeatedly and extensively abused", spending long periods in solitary confinement, and that he was "very weak".
"What is happening to this man is a disgrace," he said.
"We have welcomed the government's insistence that it wants him back in this country, but are worried that any further delays might prove fatal."
More than 100,000 people across Britain signed an online petition in support of his release from Guantanamo. Christians Against Torture supporters have added their voices to the call by writing to their MPs and to the Foreign Secretary asking him to put pressure on the US for Aamer's transfer.
Reverend Jenkins added: "We are appalled and ashamed at the way Shaker Aamer continues to be treated ... There are real fears that he might not survive this ordeal.
"He has never been charged with any offence, still less convicted, and we believe our government has an obligation to step up its demands for his return to this country to be reunited with his wife and children."