Khan's health condition started deteriorating on Saturday night, after which he was rushed to the Khan Research Laboratories Hospital in Islamabad, according to his family members.
In August, he was admitted to the same hospital after testing positive for Covid-19 and was discharged after recovery. Pakistani Prime Minister Imran Khan expressed his condolences to Khan's family, saying on Twitter that he was loved by the nation because of "his critical contribution in making Pakistan a nuclear weapon state". "For the people of Pakistan he was a national icon," he said.
Pakistani President Arif Alvi said in a tweet that the nuclear scientist has helped Pakistan "develop nation-saving nuclear deterrence, and a grateful nation will never forget his services in this regard".
However, the most of the world saw him differently. After training and working in metallurgical sciences in western Europe, Khan set up the Khan Research Laboratories in 1976 to pioneer his country's nuclear programme - set up to counter neighbouring India, which had tested a nuclear device in 1974. His work was mainly on centrifuges required to purify the uranium.
However, his maverick tendencies made him a bad team player, and his quest for publicity also worked against him. Khan, who was already in the cross-hairs following the Dutch government's accusations of stealing centrifuge designs, began transferring information to both North Korea and Iran from the late 1980s.
As Pakistan faced sanctions after testing a nuclear device in 1998, his role came under further scrutiny and in 2003, the US handed over a dossier of his proliferation activities to the then Pervez Musharraf government. On February 4, 2004, a tearful Khan appeared on national television to confess his proliferation activities and seek the nation's apologies. He was pardoned by President Musharraf the very next day, but placed under house arrest. In 2009, he successfully moved court to have the restriction withdrawn. However, he was never in the limelight again.