Documents with detailed designs for missiles, bombs and nuclear weapons were found in Kabul safe houses used by Osama bin Laden's al-Qaida network, it was reported today.

The partly-burnt papers were found by The Times in a hastily abandoned house in the Karta Parwan quarter of the city, the newspaper reported.

There are descriptions of how the detonation of TNT compresses plutonium into a critical mass, sparking a chain reaction, and ultimately a thermonuclear reaction, it said.

Documents were found in two of four al Qaida houses which has been used by Arabs and Pakistanis and even reportedly by bin Laden himself, the newspaper said.

They included studies into the development of a kinetic energy supergun capable of firing chemical or nuclear warheads, external propulsion missiles, preliminary research on the creation of a thermonuclear device, and many instructions for making smaller bombs.

The discovery of the detailed instructions, written in Arabic, German, Urdu and English, confirms the West's worst fears of an attack far worse than the September 11 atrocities, the newspaper noted.

Nuclear experts say the design suggests bin Laden may be working on a fission device similar to Fat Man, the bomb dropped on Nagasaki, but they emphasise it is extremely difficult to build a viable warhead, it added.

The houses were abandoned on Monday night as Taliban units and their allies fled the city as the Northern Alliance advanced.

Liberal Democrat foreign affairs spokesman Menzies Campbell said: "We cannot be sure that Osama bin Laden does not have nuclear weapons, but the events of September 11 tell us that he is unlikely to show any scruples about using, or threatening to use, such weapons.

"No-one should be in any doubt that, in bin Laden and al Qaida, we are dealing with the most unscrupulous enemy that modern democracy has seen."