Pakistan's top nuclear scientist has admitted he leaked nuclear secrets to groups working for Iran, Libya and North Korea, Pakistani officials say.
Khan is revered as a "hero"
Officials told journalists that Dr AQ Khan had confessed to passing on information about nuclear technology in the 1980s and 1990s.
Dr Khan was dismissed as a scientific adviser to the government on Saturday - a move that sparked an outcry.
He is regarded as a national hero for making Pakistan a nuclear power.
Four other scientists are also implicated, a senior Pakistani official told journalists at a two-hour briefing in Islamabad on Sunday night.
"Dr Qadeer [Khan] and four others have accepted that they were involved in leaking nuclear know-how outside Pakistan to groups working for Iran, Libya and North Korea," the AFP news agency quoted the unnamed official as saying.
Those present at the briefing were given an account of how Dr Khan had allegedly run a network that systematically smuggled nuclear equipment to third countries using chartered planes and shared secret designs for centrifuges capable of producing weapons-grade uranium.
Dr Khan, it is alleged, also secretly travelled abroad to explain to Iranian, Libyan and North Korean scientists how to make nuclear bombs.
The 69-year-old is under 24-hour watch and has yet to respond to the government's account, but his family have said in the past that he is being made a scapegoat.
Dr Khan was sacked on Saturday as part of investigations into the alleged illegal sale of nuclear technology to Iran and Libya but Monday's reports also name North Korea.
If the reports of his confession are confirmed, the authorities will have to decide whether to prosecute him.
The BBC's Paul Anderson in Islamabad says that will risky because of a possible domestic backlash.
A group of opposition parties has already said it will launch a nationwide campaign against what it called the harassment and humiliation of Pakistan's nuclear scientists by the government.
They accuse President Pervez Musharraf of bowing to American pressure over the move.
Khan is credited with giving Pakistan a nuclear deterrent
According to the official briefing, the Pakistani Government, military and security services were not involved and - if the state was guilty of anything - it was a security lapse.
But Dr Khan's supporters say - if there was nuclear transference - it could not have happened without the knowledge of military intelligence.
Qazi Hussein Ahmed, the head of the main religious party, Jamaat-e-Islami, said government moves against Dr Khan are both unjustified and suspicious:
"The Pakistani people think that he is humiliated because of the American pressure," he said.
The inquiry began two months ago after the UN gave Pakistan information it had gathered about Iran and Libya's nuclear programmes.
More than 15 people from the country's premier nuclear enrichment facility, Khan Research Laboratories (KRL), have been questioned so far and five scientists and officials are still in the custody of the authorities.
Observers say allegations about illegal sales in the Pakistani press make the country's future as a responsible nuclear power look vulnerable.
Dr Khan had held the post of scientific adviser since retiring as head of the country's top nuclear facility in 2001.