The discovery of designs for a compact nuclear bomb has raised fears that Iran and North Korea might have obtained blueprints enabling them to mount long-range strikes with nuclear-armed missiles.

Designs for a nuclear device small enough to fit on a ballistic missile were found on computers linked to the international smuggling ring that supplied nuclear technology to Iran, Libya and North Korea, a top US expert says.

“These advanced nuclear weapon designs may have long ago been sold off to some of the most treacherous regimes in the world,” David Albright, the president of the Institute for Science and International Security, wrote in a report provided yesterday to The Washington Post.

The blueprints were among some 30,000 heavily encrypted documents found in 2006 on computers linked to the now-defunct smuggling ring run by Abdul Qadeer Khan, the “father” of Pakistan’s atomic weapons project.

The Swiss President, Pascal Couchepin, said last month that the documents had been destroyed under the supervision of the International Atomic Energy Agency to prevent them getting into the hands of a terrorist organisation or an unauthorised state. Mr Couchepin said: “There were detailed construction plans for nuclear weapons, for gas ultra-centrifuges to enrich weapons-grade uranium and for guided missile delivery systems.”

The computers belonged to three members of a Swiss family under investigation for their alleged role in the “A. Q. Khan network”, which was broken up in 2003. Dr Khan was placed under house arrest. Two brothers, Urs and Marco Tinner, are being held on suspicion of violating export controls, while their father, Friedrich, was also detained but has since been released.

The case is complicated by reports that Urs Tinner cooperated with the CIA in exposing nuclear trafficking to Libya. The Zurich weekly Sonntags Zeitung said in March that Urs Tinner provided the tip that led to the seizure in 2003 of the German-registered freighter BBC China as it carried components for a uranium enrichment plant from Dubai to Libya.

The interception prompted Colonel Muammar Gaddafi, the Libyan leader, to abandon his country’s covert efforts to acquire nuclear weapons.

The A. Q. Khan network is known to have supplied Libya with designs for a bulky Chinese nuclear weapon from the mid1960s. The designs discovered on the Tinners’ computers were for a smaller device similar to those for a Pakistani nuclear bomb.

The New York Times reported that the blueprints were for a bomb that was half the size and had twice the power of the Chinese weapon, with far more modern electronics.

“These would have been ideal for two of Khan’s other major customers, Iran and North Korea,” Mr Albright wrote. “They both faced struggles in building a nuclear warhead small enough to fit atop their ballistic missiles and these designs were for a warhead that would fit.”

Because the designs are in digital form, it would be easy to distribute copies. It is not known if the bomb designs were sent to Iran, North Korea or any other nation.

Mr Albright said that the IAEA confronted the Pakistani Government shortly after the discovery. Its officials “were genuinely shocked”.

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