Fahim, 73, a soft spoken politician from southern Sindh province was diagnosed with blood cancer recently and has been in critical condition after he also sought treatment abroad.
He remained loyal to the Pakistan Peoples Party (PPP) throughout his political career turning down several offers for top positions from different governments including military ruler Pervez Musharraf.
Fahim was also once a front runner for the prime ministership in 2002 when he was offered by the then President Muharraf.
Since last month he remained under treatment in Karachi.
Fahim who was the spiritual leader of the Sarwari Jamaat (a Sufi sect) in his hometown of Hala was a member of the National Assembly from Hala.