Mohammad Asghar, 71, from Edinburgh, was arrested in 2010. He has been deprived of medicine and held in a cell with several other prisoners despite his documented history of illness that has been supported by an affidavit from his doctor in Scotland.
He was sectioned in this country after being diagnosed with paranoid schizophrenia and four months after his release - following a month under observation in the Royal Victoria Hospital - he travelled to Pakistan, where the alleged offences took place.
A court in Rawalpindi, Pakistan sentenced Mr Asghar to death after convicting him of blasphemy on Thursday.
Human rights charity Reprieve and Scots lawyer Aamer Anwar called on the UK Government to act to halt the process - although it is also feared a lengthy appeal could follow - while Foreign and Commonwealth Office Minister Baroness Warsi condemned the court decision.
The Pakistan High Commission in London last night admitted that Mr Asghar's mental health may not have been raised during proceedings. A spokesman said therefore his "conduct in the court must have been responsible for the sentence."
It said the matter was being investigated and Mr Asghar's lawyers would be able to "forcefully plead his case" on the basis of his previous history in UK.
A spokesman stressed the Pakistani Government was anxious an innocent person was not the victim of a 'misplaced judicial process' and said it was committed to fair justice.
Mr Asghar's legal team at Reprieve obtained information confirming his mental and physical ill health and risk of suicide.
These include his NHS records and the affidavit signed by Dr Jane McLennan, senior consultant at Royal Victoria Hospital in Edinburgh. Ms McLennan said: "Were Mr Asghar allowed to return to the UK, I would be happy to have him admitted to my care."
Maya Foa of Reprieve said: "The evidence is clear that he is unable to defend himself in court.
"Worse still, he is currently being held in utterly unsuitable conditions in prison, and we are very concerned about his health."
Baroness Warsi said: "We will be raising our concerns in the strongest possible terms."