Lawyers for a British prisoner shot in Pakistan said the Prime Minister must intervene after reports he is to be returned to jail.
Mohammad Asghar, 70, was targeted in Adiala prison in Rawalpindi, where he faced the death penalty after being convicted under blasphemy laws.
Legal action charity Reprieve said Mr Asghar, who suffers from paranoid schizophrenia, must remain in a secure medical facility where he can receive treatment in safety.
Spokeswoman Maya Foa said: "To move Mr Asghar to a prison now, where there is no way of guaranteeing his safety, would be to effectively hand out a second death sentence that could be carried out at any moment.
"The British Government simply cannot allow this to happen.
"They must ensure that he is held in a secure medical facility where he can receive the treatment he so desperately needs, both for his recent injuries and his existing mental illness."
Mr Asghar, from Edinburgh, was jailed in January after writing letters to a number of people claiming to be the Prophet Mohammed.
He received surgery after he was shot in the back by a guard on Thursday morning and remains in hospital.
People accused or convicted of blasphemy in Pakistan face a high level of risk from attacks by religious extremists, according to Reprieve.
An appeal has been lodged against his conviction, with lawyers pointing out that judges did not consider evidence of his mental illness.
Family solicitor Aamer Anwar said: "The Asghar family is horrified to learn that their father is being threatened with being returned to prison tomorrow morning.
"The Pakistani authorities are unable to guarantee Mr Asghar's security and we have insisted that he be moved to a secure military medical facility where he can continue to be treated in safety.
"The repeated failure of David Cameron to intervene is a green light for the Pakistani authorities to continue punishing Mr Asghar. When will somebody in power act to save his life?"
Mr Ashgar's daughter Jasmine Rana, 40, last week issued a plea to the British Government to do everything it could to ensure her father's safety.
The campaign has been backed by Stephen Fry, who tweeted today: "Govt must not let mentally ill Brit Mohammed Asghar be returned to prison. They must do all they can to get him home."
Frankie Boyle also tweeted: "70yo Scotsman Mohammed Asghar shot, under PK death sentence for blasphemy, surely government should get him home."
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