The report has already led to security being tightened at all laboratories run the Department of Energy, including those at Los Alamos and Livermore. Fears over the new security regime - visitors from "sensitive" countries such as India, China and Russia now require special permission from Bill Richardson, head of the DOE, to attend the laboratories - have already led several foreign-born researchers to turn down post-doctoral positions at Los Alamos. More than half the current post-docs at Los Alamos were born outside the US.

The Cox report also reveals that security leaks allowed China to build and test a so-called neutron bomb - a device which kills people but leaves buildings intact - in 1988. As well as acquiring plans to all the main US warheads, China has also obtained the simulation software used to model nuclear explosions.

During 1996 China declared it would stop nuclear testing and sign the Comprehensive Test Ban Treaty. But signing the treaty would hinder China's efforts to develop more sophisticated and smaller nuclear weapons, hence its interest in stealing secrets from the US. "It would have been virtually impossible for the People's Republic of China to fabricate and test successfully small nuclear warheads prior to its 1996 pledge to adhere to the Comprehensive Test Ban Treaty, " says the report.

Two physicists, Wen Ho Lee and Peter Lee, are accused in the report of leaking details of nuclear weapons and advanced radar systems to the Chinese. Peter Lee was arrested and sentenced to 12 months in prison for providing details of synthetic-aperture radar systems. Such systems can be used to detect nuclear submarines. Meanwhile, Wen Ho Lee transferred thousands of classified documents on the US nuclear weapons programme from a secure computer to one accessible from the Internet. China is believed to have downloaded files from this system. The case against Wen Ho Lee was dropped due to lack of evidence, although he was later fired by Los Alamos for breaching security precautions and failing a lie detector test.

The Cox report highlights how China not only relied on spies at the US laboratories, but also on gathering information piecemeal by reviewing unclassified publications and by interacting extensively with nuclear weapon scientists outside the laboratories. China also managed to obtain a large amount of 'dual-use' technology - which could be adapted from civilian to military use - by using over 3000 so-called front companies. And two US companies, Loral Space Systems and Hughes Electronics, are heavily criticised in the report for providing the Chinese space programme with information that was subsequently used to improved the reliability of China's missile programme.