Pakistani nuclear scientist Abdul Qadeer Khan died on Sunday morning at a hospital in Islamabad at the age of 85.
Dawn reported Khan had been moved to the hospital with lung problems.
Khan, who was born in Bhopal, moved to Pakistan in 1952. After his graduation, Khan moved to what was then West Germany and later the Netherlands and Belgium to pursue advanced studies in materials technology and metallurgy. He received a PhD in metallurgical engineering from Catholic University of Leuven in Belgium in 1972.
Following India's first nuclear explosion in 1974, Khan wrote to then Pakistan prime minister Zulfiqar Ali Bhutto to offer his expertise.
Khan, who was working in the Netherlands, left for Pakistan in December 1975, allegedly with copied blueprints for centrifuges. In Pakistan, Khan worked to boost the country's uranium enrichment capabilities in the 1970s to boost its nascent nuclear weapons capability.
Khan was under surveillance of Dutch intelligence agencies who believed he was involved in suspicious activities. He was convicted in absentia by a Dutch court in 1983 for nuclear espionage and sentenced to four years in prison.
In the early 2000s, Khan was accused of seeking to provide nuclear technology to a variety of countries such as Iraq, Iran, North Korea and Libya. In February 2004, Khan 'confessed' on Pakistani television he had engaged in illegal nuclear dealings. He was immediately pardoned by the regime of Pervez Musharraf. However, analysts believe Khan had been made a scapegoat for illegal dealings by the Pakistani government.
In 2009, then US secretary of state Hillary Clinton described Khan as "probably the world's greatest proliferator" who had done "incalculable" damage.
After news of his death, Pakistan president Dr Arif Alvi tweeted he personally knew Khan. He wrote, "He helped us develop nation-saving nuclear deterrence, and a grateful nation will never forget his services in this regard."
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Pakistan Prime Minister Imran Khan tweeted, “Deeply saddened by the passing of Dr A Q Khan. He was loved by our nation bec of his critical contribution in making us a nuclear weapon state. This has provided us security against an aggressive much larger nuclear neighbour. For the people of Pakistan he was a national icon.”
Shehbaz Sharif, chief of the PML-N and brother of former prime minister Nawaz Sharif tweeted, "Today the nation has lost a true benefactor who served the motherland with heart and soul. The passing of Dr Abdul Qadeer Khan is a huge loss for the country. His role in making Pakistan an atomic power remains central. May Allah shower his blessings on his soul!" Nawaz was prime minister when Pakistan conducted its nuclear tests in 1998.