Indian national Sarabjit Singh, who was severely injured in a brutal assault last week, has slipped into a "non-reversible" coma.
Doctors treating the India prisoner in a Lahore hospital are struggling to save his life, official sources said on Wednesday.
The deterioration in Sarabjit's condition could lead to "brain death", the sources told PTI.
His measurements on the Glasgow Coma Scale, which indicates the levels of consciousness and damage to a person's central nervous system, had dropped to a "critical level", the sources said.
A source said Sarabjit's heart is beating "but without brain function" because of the extensive head injuries he sustained when he was assaulted by prisoners at Kot Lakhpat Jail in Lahore on Friday.
Sarabjit is completely unresponsive and unable to breathe without ventilator support.
The sources said the medical board supervising Sarabjit's treatment was not in a position to declare him brain dead without consulting his family and Pakistani authorities.
The sources further said Sarabjit could not be removed from the ventilator without his family's consent and the approval of the Pakistani government.
Sarabjit's wife Sukhpreet Kaur, daughters Poonam and Swapandeep Kaur and sister Dalbir Kaur, who went to Lahore on Monday to see him, returned to India on Wednesday.
On Tuesday, doctors treating Sarabjit at the state-run Jinnah Hospital said that his condition had further deteriorated though he had not been declared brain dead.
They said his blood pressure was being maintained with support and his chances of survival were very slim.
Sarabjit, 49, sustained several injuries, including a fractured skull, when six prisoners attacked him in jail on Friday afternoon. He was hit on the head with bricks and has been comatose in hospital since then.
He was convicted of alleged involvement in a string of bomb attacks in Punjab province that killed 14 people in 1990.
His mercy petitions were rejected by the courts and former President Pervez Musharraf.
The outgoing Pakistan People's Party-led government put off Sarabjit's execution for an indefinite period in 2008.
Sarabjit's family says he is a victim of mistaken identity and had inadvertently strayed across the border in an inebriated state.