A Pakistani policeman is accused of shooting two prison inmates accused of blasphemy, killing one and wounding another on death row.

Local Christian pastor Zafar Bhatti was fatally shot, while Briton Muhammad Asghar, who has been sentenced to death, was wounded in the shooting in Adiyala prison in Rawalpindi.

Asghar, from the Scottish city of Edinburgh, was arrested in 2010 and sentenced to death in January after a disgruntled tenant presented letters he had written saying he was a prophet.

The 70-year-old had previously been detained under the mental health act in Britain and diagnosed with paranoid schizophrenia, according to documents his lawyers supplied to Reuters.

"A jail employee shot him (this) morning. He is in a stable condition," an official of Adiyala Prison in Rawalpindi told AFP.

"The employee has been arrested and investigations have been launched," the official said, requesting anonymity as he was not authorised to speak to media.

A member of Asghar's legal team, who also requested anonymity, told AFP their client was shot in the back and a criminal case had been started against a constable.

The British High Commission in Islamabad said in a statement: "We can confirm that a British national has been injured in prison in Pakistan. We are providing consular assistance."

Asghar's family has urged the British government to intervene to bring him home, saying he had attempted suicide in jail.

British prime minister David Cameron earlier said he was "deeply concerned" about the issue.

Inmate threatened before 'barbaric' fatal shooting

Christian pastor Bhatti was on trial after an Islamic leader accused him in 2012 of sending text messages deemed derogatory to the Prophet Mohammed's mother.

His family said police investigations showed the phone was registered to someone else.

In recent weeks, Bhatti had received death threats in prison from both inmates and guards, his family told Pakistan-based human rights group Life for All.

He was being held in the same cell as Asghar.

"This is a barbaric act. There had been threats. The court should have instructed police to ensure Bhatti's safety," said Xavier Williams of rights group Life for All.

"Killing of a person who was falsely accused is mockery of the judicial system. The protectors of the innocent have become the predators."

Bhatti was known for working to protect the human rights of Pakistan's beleaguered Christian minority.

Blasphemy is punishable by death in Muslim-majority Pakistan.

Last week, gunmen shot dead a university professor in Karachi known for his liberal views on Islam who had been labelled a "blasphemer" in a text message campaign.

Adiyala jail also houses Mumtaz Qadri, the former bodyguard of Punjab provincial governor Salman Taseer whom he gunned down in 2011 over the politician's call for the blasphemy laws to be reformed.

Qadri was feted by a wide segment of the population including many lawyers, while a mosque named in his honour was recently built in the capital Islamabad.

Though Asghar was on death row, Pakistan has had a de facto moratorium on civilian hangings since 2008.

Only one person has been executed since then, a soldier convicted by a court martial and hanged in November 2012.

AFP/Reuters