Kirpal Singh was accused of a role in a bombing at the Faisalabad Railway Station in 1991 and was sentenced to death by a Pakistani court for spying and terrorism.
Pakistan today told India that the death of an Indian in a jail in Lahore on Monday was caused by a heart attack.
New Delhi had asked its envoy to take up "at the highest level" the death of 50-year-old Kirpal Singh at the Kot Lakhpat jail and the handover of his remains for a cremation.
Kirpal Singh spent nearly 25 years at the jail where a more prominent prisoner, Sarabjit Singh, died in 2013 after a brutal attack by other inmates.
Kirpal Singh was accused of a role in a bombing at the Faisalabad Railway Station in 1991 and - like Sarabjit - was sentenced to death by a Pakistani court for spying and terrorism.
As his family, friends and supporters protested near the border today, India's acting High Commissioner JP Singh met a foreign ministry official in Islamabad.
"According to the government of Pakistan, Kirpal Singh died on April 11 at 1455 hours due to heart attack. We await further details," External Affairs Ministry spokesperson Vikas Swarup told reporters.
Kirpal Singh was arrested in 1992 after he allegedly crossed over to Pakistan through the Wagah border.
A resident of Gurdaspur in Punjab, Kirpal Singh was reportedly later acquitted of the charges by the Lahore High Court but his death sentence could not be commuted for reasons not known.
Kirpal Singh's sister Jagir Kaur said the family could not fight for his release because of the lack of resources and political support.
Sarabjit Singh died days after he was attacked by two other inmates.
"My brother Kirpal has been murdered just like Sarabjit was earlier. The Pakistani jail authorities are responsible for his death," said Jagir Kaur, protesting at the Attari-Wagah border between the two countries.
Dalbir Kaur, the older sister of Sarabjit Singh, joined Kirpal Singh's family in the protests.
The family wants that Kirpal Singh's body be handed over to them for cremation at his village.