Pakistan's Supreme Court dismissed an appeal to stop the hanging of a death row prisoner diagnosed with a mental illness, officials said on Tuesday.

Imad Ali, said to be around 50, was sentenced to death for murdering a religious cleric in 2002 but was later diagnosed with paranoid schizophrenia, said Justice Project Pakistan, a charity providing him legal aid.

He was due to be hanged on September 20 after a court in the central province of Punjab issued his death warrants.

At the last moment, the Supreme Court in Islamabad delayed the execution for a week on medical grounds, the charity said.

But judges on Tuesday refused an appeal for a further delay, court official Shahid Hussain said.

Reprieve, a London-based legal campaign group, said Ali can now be hanged within a week.

Pakistani authorities lifted a six-year unofficial moratorium on the death penalty after Islamist militants killed over 150 people, mostly children, at a school in the north-western city of Peshawar in December 2014.

More than 400 of an estimated 8,000 death row prisoners have since been hanged in the country, according to statistics compiled by the Human Rights Commission of Pakistan.

Pakistan is signatory of The Convention on the Rights of Persons with Disabilities (CPRD), an international convention that guarantees the dignity of individuals with disabilities.