Pakistani scientist AQ Khan admitted leaking nuclear secrets in 2004
An international smuggling ring managed to acquire blueprints for an advanced nuclear weapon, a former UN arms inspector is to say in a new report.
David Albright, who investigated the ring led by Pakistani scientist AQ Khan, found the drawings in 2006.
His report, due to be published later this week but seen in advance by the Washington Post, suggests the plans may have been sold to rogue regimes.
The blueprints included key details for building a compact nuclear device.
Such a device, unlike less advanced ones, could be fitted to the kind of ballistic missile used by Iran and more than a dozen developing countries.
In 2004, Dr Khan admitted having passed on nuclear secrets to Libya, Iran and North Korea before his network was dismantled.
However, in a BBC interview last month, Dr Khan said that the allegations were false and claimed he had been pressured into confessing "in the national interest".
In his report, Mr Albright states that he found the drawings on computers owned by Swiss businessmen, the Washington Post newspaper says.
The computer contents have since been destroyed by the Swiss authorities under the supervision of the UN nuclear watchdog, the International Atomic Energy Agency.
But UN officials cannot rule out the possibility that the blueprints were shared with others before they were found, Mr Albright says.
"These advanced nuclear weapons designs may have long ago been sold off to some of the most treacherous regimes in the world," the Post quotes him saying in his report.
The computers on which the drawings were found belonged to Swiss businessmen Friedrich Tinner and his sons Marco and Urs.
They are awaiting trial in Switzerland on charges connected to their alleged involvement in the smuggling ring.
Speaking in May, after the destruction of the computer files was disclosed, Swiss President Pascal Couchepin said the action had been taken to prevent the information falling "into the hands of a terrorist organisation" or a rogue state.
He told reporters: "There were detailed construction plans for nuclear weapons, for gas ultracentrifuges to enrich weapons-grade uranium, as well as for guided missile delivery systems."
The files were among information seized in the course of a government investigation into the Tinners that began in 2004.
Mr Albright's report is the first to allege that the documents included plans for a more advanced nuclear weapon.