Blueprints for a sophisticated and compact nuclear warhead have been found in the computers of the world's most notorious nuclear-smuggling racket, according to a leading US researcher.

The digital designs, found in heavily encrypted computer files in Switzerland, are believed to be in the possession of the US authorities and of the International Atomic Energy Agency, in Vienna, but investigators fear they could have been extensively copied and sold to "rogue" states via the nuclear black market.

David Albright, a physicist, former UN weapons inspector and authority on the nuclear smuggling ring run by the Pakistani metallurgist Abdul Qadeer Khan, said the "construction plans" included previously undisclosed designs for a compact warhead that could fit on Iran's medium-range ballistic missiles.

"These advanced nuclear weapons designs may have long ago been sold off to some of the most treacherous regimes in the world," wrote Albright.

The Khan network was exposed in 2003, having been found to have supplied clandestine nuclear projects in Iran, North Korea, and Libya. Albright has been investigating the network ever since. The racketeers are known to have provided Libya with an older, cruder, bomb design.

Albright said the network might have supplied Tehran or Pyongyang with the more advanced and much more useful bomb blueprints that have now surfaced. "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," he stated in a report to be published this week and which was leaked to the Washington Post. "These would have been ideal for two of Khan's other major customers, Iran and North Korea."

The disclosures about the new weapons design arose from a Swiss investigation into engineers awaiting trial for alleged involvement in the Khan network.

The Guardian reported in May that nuclear investigators and experts were alarmed that extremely sensitive information from the Swiss computers - including warhead designs and details on making weapons-grade uranium - were circulating on the nuclear black market.

In recent months the Swiss government has secretly destroyed 30,000 files and documents from the computers of Urs Tinner, a Swiss engineer said to be heavily involved with the Khan operations and also alleged to have spied for the CIA. Tinner has been in custody for four years awaiting trial. His brother Marco is also in custody, while his father, Friedrich, whose relationship with Khan goes back almost 30 years, has been arrested and released.

"This was very proliferation-sensitive stuff," said a western diplomat.

The Swiss president, Pascal Couchepin, announcing the destruction of the files last month, said: "There were detailed construction plans for nuclear weapons, for gas ultracentrifuges to enrich weapons-grade uranium as well as for guided missile delivery systems."

Albright found the blueprints included designs for a compact warhead that could fit on Pyongyang's medium-range Nodong rockets as well as on the Iranian Shahab-III missile.

Five years ago a boatload of uranium-enrichment equipment destined for Libya was intercepted by US and British intelligence, and Libya's leader, Colonel Muammar Qadafy abandoned his illicit nuclear project. That development led to the exposure of the Khan network: when CIA and MI6 agents examined the Libyan material they found designs, supplied by the Khan network, for an older, larger, and simpler nuclear bomb of Chinese vintage. The designs were put under IAEA seal and taken to the US.

The new designs, running to hundreds of pages, were found in 2006 when the Swiss managed to break the codes on the Tinner computers. Also that year, a German court heard testimony claiming Tinner had told investigators he had nuclear bomb construction plans at his office in eastern Switzerland. The designs were in digital form and believed to have been copied on to the network's computers in Dubai, which served as the hub for the Khan operations.

The testimony surfaced at the trial in Germany of Gotthard Lerch, a German engineer also alleged to have played a central role in the Khan network. The trial was quickly halted because of procedural irregularities, but Lerch was put back on trial last week in Stuttgart and evidence from the Tinner investigation is likely to be used in the case.

The Swiss told the IAEA and the Americans about their find in 2006. Officials from the Vienna agency and Washington supervised the recent destruction of the Swiss files.

But an expert on the nuclear network, Mark Fitzpatrick, of the International Institute for Strategic Studies, said he was certain copies of the blueprint had been made and that no one knew where they were.

According to Albright, the advanced weapons design is similar to a Pakistani bomb design. Khan has been under house arrest in Islamabad since "confessing" in 2004, though there are moves to get him released. The scientist is seen as a national hero in Pakistan.

In his first interview with the western media last month, Khan told the Guardian his confession was not genuine; he specifically mentioned the Swiss case to emphasise how easy it would be for any country to satisfy its nuclear bomb ambitions.

International interrogation of Khan might clarify the provenance and whereabouts of the new bomb blueprint, but the Pakistanis have refused the US and the IAEA investigators access to Khan.