The deterioration in 49-year-old Sarabjit's condition could lead to "brain death", official sources told PTI in Lahore adding he has slipped into "non-reversible" coma.
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.
Meanwhile, Indian High Commissioner to Pakistan Sharat Sabharwal today met Pakistani Foreign Secretary Jalil Abbas Jilani and sought immediate release of Sarabjit.
"We are concerned at the condition of Sarabjit Singh indicated by reports made available by doctors treating him in Jinnah Hospital (in Lahore).
"Our High Commissioner has met the Pakistan Foreign Secretary and urged the Pakistan government to immediately release him on humanitarian and sympathetic grounds so that he can benefit from the best available treatment in India," Spokesperson in the Ministry of External Affairs Ministry said in New Delhi.
"This is not the time for invoking legal and bureaucratic reasons for not taking the right steps to save a human life. We believe that every endeavour should be made to save his life," the spokesperson said.