Qamar David had been awaiting appeal against his 2010 conviction but was found dead today in his cell in Karachi's main prison.

Prison authorities have attributed David's death to a heart attack but this has been called into question by church leaders and human rights activists who are calling for an independent autopsy.

Andrew Johnston, Advocacy Director of Christian Solidarity Worldwide (CSW), said: "Given the threats known to be faced by blasphemy prisoners, it is imperative that the true cause of his death be investigated properly and independently."

David had been in prison since being accused of committing blaspheming against the Prophet Muhammad in 2002.

News of his death has sparked particular concern, coming just two weeks after the assassination of Pakistan's Minorities Minister Shahbaz Bhatti, an outspoken critic of the blasphemy laws.

Mr Johnston said David's death underlined the urgent need to continue Bhatti's work.

"The government must address both the ease with which the blasphemy laws can be abused, and the social attitudes which view it as acceptable to do so,” he said.

David's lawyer had been seeking an appeal hearing ever since a sessions court in Karachi sentenced David to life imprisonment in February last year.

According to CSW, David's lawyer has always maintained that the allegations were spurious and triggered by a business rivalry, and that the conviction was the result of pressure from local religious clerics and their supporters.

The appeal process can take years in Pakistan and David and his lawyers had been on the receiving end of death threats and intimidation at court hearings connected to David's case.

Mr Johnston said: “We are shocked to hear this sad news and our thoughts and prayers are with Qamar David’s wife and children.

"The last nine years of this family’s life have been utterly ravaged by the consequences of a vindictive blasphemy accusation that would have very likely been dismissed by the High Court in time.

"It is yet another tragic example of lives needlessly destroyed by the blasphemy laws in Pakistan and the inability of the government, court system and prisons to prevent this."

David's death has also been condemned by the Centre for Legal Aid, Assistance and Settlement (CLAAS), which was involved in the early stages of David's case.

CLAAS' UK Coordinator Nasir Saeed said: “Qamar is yet another victim of Pakistan’s unjust blasphemy laws.

"Even if the charges brought against him were true, it is appalling that he should have been sentenced to life in prison and even more tragic that he should have died there.

"In the past, Christians have been attacked in prison or died behind bars in suspicious circumstances.

"His death raises more questions about the safety of Christians in prison and we hope that that there is no foul play here. Our thoughts and prayers are with Qamar’s family.”