Revered as the father of Pakistan's nuclear bomb, Khan was lauded for bringing the nation up to par with arch-rival India in the atomic field and making its defences "impregnable".

But he found himself in the crosshairs of controversy when he was accused of illegally proliferating nuclear technology to Iran, Libya and North Korea.

Khan was placed under effective house arrest in the capital Islamabad in 2004 after he admitted running a proliferation network to the three countries.

In 2006, during his house arrest, Khan suffered from prostate cancer, but recovered after surgery.

A court ended his house arrest in February 2009, but Khan has to inform authorities of his movements in advance, even within Islamabad, with security accompanying him on his every step.

Born in Bhopal, India on April 1, 1936, Khan was just a young boy when his family migrated to Pakistan during the bloody 1947 partition of the sub-continent at the end of British colonial rule.

He did a science degree at Karachi University in 1960, then went on to study metallurgical engineering in Berlin before completing advanced studies in the Netherlands and Belgium.

The 81-year-old's crucial contribution to Pakistan's nuclear programme was the procurement of a blueprint for uranium centrifuges, which transform uranium into weapons-grade fuel for nuclear fissile material.

He was charged with stealing it from The Netherlands while working for Anglo-Dutch-German nuclear engineering consortium Urenco, and bringing it back to Pakistan in 1976.

On his return to Pakistan, then prime minister Zulfikar Ali Bhutto put Khan in charge of the government's nascent uranium enrichment project.

By 1978, his team had enriched uranium and by 1984 they were ready to detonate a nuclear device, Khan later said in a newspaper interview.

The 1998 nuclear test saw Pakistan slapped with international sanctions and sent its economy into freefall.

Khan's aura began to dim in March 2001 when then president Pervez Musharraf, reportedly under US pressure, removed him from the chairmanship of Kahuta Research Laboratories and made him a special adviser.

But Pakistan's nuclear establishment never expected to see its most revered hero subjected to questioning.

The move came after Islamabad received a letter from the International Atomic Energy Agency, a UN watchdog, containing allegations that Pakistani scientists were the source of sold-off nuclear knowledge.

Khan said in a speech to the Pakistan Institute of National Affairs in 1990 that he had shopped around on world markets while developing Pakistan's nuclear programme.

"It was not possible for us to make each and every piece of equipment within the country," he said.