Abdul Qadeer Khan, the controversial pioneer of Pakistan's nuclear programme, was on Tuesday diagnosed with prostate cancer, reports said.
The cancer was detected during a routine medical test in early August and confirmed by a detailed examination at the Khan Research Laboratory hospital near Islamabad, The News reported without identifying the source.
"The results have unfortunately indicated adino-carcinoma (cancer) of prostate. Further investigations are being conducted by a board of doctors," it said in its news update.
Khan has been under house arrest since he admitted to passing on the nuclear technology and designs that could help others build nuclear weapons.
The alleged recipients, as per investigations and media reports at home and in the West, are North Korea, Libya, Iran and Al-Qaeda chief Osama bin Laden.
The scientist has denied the allegations while Pakistan's stand that he did it all on his own and that the successive governments over three decades were unaware is seriously contested by experts.
So close has been Khan's security while in detention that his relatives were disallowed from seeing him some weeks ago. He has since lived in virtual isolation, reports have indicated.
Born in Bhopal, India, Khan remains the national hero for his pioneering role in helping Pakistan build nuclear capability.
President Pervez Musharraf, on whose orders he has been detained, has himself described Khan as "my hero".
Musharraf's action followed pressures from the US after investigations showed that Khan had been engaged in passing on nuclear secrets and was in touch with Al-Qaeda.
Khan himself stole the technology and designs while working for a laboratory in The Netherlands in the mid-1970s.
The US has sought access to Khan for interrogation, but Pakistan has protested and not allowed any foreigner to see him for this purpose.
There were reports this June that the US might renew its demand. But Pakistan maintained that it treated the chapter as "closed".