Dr Khan played the key role in developing Pakistan's nuclear military capability, which culminated in successful tests in May 1998.

Coming shortly after similar tests by India, Dr Khan's work helped seal Pakistan's place as the world's seventh nuclear power and sparked national jubilation.

In March 2001 he was promoted to the inner circle of the country's military leadership as special science and technology adviser to President Pervez Musharraf.

He was sacked from the position unceremoniously in January 2004 during the investigation.

But revelations that he has passed on nuclear secrets to other countries have shocked and traumatised Pakistan.

In a televised address, Mr Khan offered his "deepest regrets and unqualified apologies".

"I take full responsibility for my actions and seek your pardon," he said.

Abdul Qadeer Khan was born into a modest family in Bhopal, India, in 1935.

He migrated to Pakistan in 1952, following the country's partition from India five years earlier.

He graduated from the University of Karachi before moving to Europe for further studies in West Germany and Belgium.

In the 1970s, he took a job at a uranium enrichment plant run by the British-Dutch-German consortium Urenco.

But in 1976, Dr Khan returned home to head up the nation's nuclear programme with the support of then prime minister Zulfikar Ali Bhutto.

During his work, Dr Khan insisted that the programme had no military purpose, but following the 1998 tests admitted: "I never had any doubts I was building a bomb. We had to do it."

He went on to work on the successful test-firings of the nuclear-capable Ghauri I and II missiles.

As he was carrying out his programme, Dr Khan was also being investigated in the Netherlands for taking enrichment technology during his time in the country.

In 1983, he was sentenced in absentia to four years in prison by an Amsterdam court for attempted espionage, although the sentence was later overturned on appeal.

Dr Khan's facility, Khan Research Laboratories at Kahuta, became Pakistan's main nuclear weapons laboratory where uranium was enriched.

It has continued to attract US suspicion and in 2003 Washington imposed sanctions on the firm for the alleged transfer of missile technology from North Korea.

In later years, Dr Khan has launched a campaign against illiteracy and built educational institutes in Mianwali and Karachi.