Disgraced Pakistani nuclear scientist Abdul Qadeer Khan is suffering from deep vein thrombosis, and his condition has deteriorated in the past few days, his wife said in Saturday.
Khan, 70, was placed under house arrest in early 2004 after admitting to selling nuclear secrets and materials to Iran, North Korea and Libya, and has been recovering from surgery for prostrate cancer in early September.
"His condition has deteriorated from what it was compared to a few days ago," his wife, Henny Khan, said.
She said her husband had been diagnosed as suffering from deep vein thrombosis, a blood clot, usually in a vein in the lower leg that can prove lethal.
"He is being treated and we have to see how it goes," she said.
Pakistan's chief military spokesman, Major General Shaukat Sultan, declined to comment on Khan's health.
Khan's role at the centre of a nuclear black market remains a source of deep embarrassment to Pakistan, which became a nuclear weapons state and the first Muslim country to build an atomic bomb in 1998.
While the United States has offered to help rival India with its civil nuclear programme, Washington has denied a similar deal for Pakistan due to its poor record on nuclear proliferation.
Despite his fall from grace, Khan is still revered by many Pakistanis as the father of their nation's atomic bomb, and several opposition parties, including the six party religious alliance, Muttahida Majlis Amal (MMA), have demanded his release.
President Pervez Musharraf says he had harboured suspicions about the source of Khan's wealth well before the CIA presented him with evidence of Khan's black market activities.
In his autobiography released last month, Musharraf said Khan had transferred nearly two-dozen P-1 and P-II centrifuges to North Korea.
He said Khan had also supplied North Korea with a flow meter, oils for centrifuges, and provided coaching on centrifuge technology, and visits to centrifuge plants.
Pakistan has pointed out, however, that its own nuclear programme used uranium, whereas North Korea appeared to use plutonium for the nuclear test carried out earlier this month.
Pakistan has refused to give US investigators direct access to Khan, but says it has shared all information about Khan's network.
The United States says it would like answers to a number of other questions, though it is confident that Khan's network has been shut down.
Pakistani experts were still asking Khan questions lodged by the International Atomic Energy Agency, the European Union and countries including Japan and South Korea, a senior Pakistani military official said in Washington last week.