The sources told PTI that Sarabjit's condition continued deteriorating through the day and his blood pressure was being maintained with support.
Sarabjit had entered a "critical phase" and his chances of survival were very slim, the sources said.
The doctors treating Sarabjit had brought the deterioration in his condition to the notice of Pakistani authorities and his family, the sources said.
Earlier in the day, Allama Iqbal Medical College principal Mahmood Shaukat, the head of a four-member medical board this is supervising Sarabjit's treatment, too confirmed that his condition had deteriorated.
"He continues to be serious but has not been declared brain dead," Shaukat told PTI.
"In our latest investigation, there was no sign of improvement (in Sarabjit's condition). Rather, his condition further deteriorated," he said.
Sarabjit is in a special intensive care unit of the state-run Jinnah Hospital.
The doctors had changed some medications after the deterioration in his condition.
A second CT scan on Sarabjit too did not reveal any signs of improvement, Shaukat said.
"The Glasgow Coma Scale of the patient is being monitored on a daily basis," he added.
A source had earlier said that Sarabjit's condition was measured as 5 on the Glasgow Coma Scale (GCS), which indicates the level of damage to a person's central nervous system. The lowest possible GCS score is 3 while the highest is 15.
The GCS assesses level of consciousness after a profound head injury and Sarabjit's reading indicated deep unconsciousness, making his treatment a major neurosurgical challenge for the medical board.
Sarabjit's sister Dalbir Kaur has demanded that Indian doctors should be called in to assist in his treatment.
Two officials of the Indian High Commission also visited Jinnah Hospital tonight and met members of the medical board to discuss Sarabjit's treatment.
Sarabjit, 49, sustained several injuries, including a skull fracture, when six prisoners attacked him in Kot Lakhpat Jail on Friday afternoon.
He was hit on the head with bricks and his neck and torso cut with sharp weapons.
Sarabjit 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 the victim of mistaken identity and had inadvertently strayed across the border in an inebriated state.