In the field of nuclear warheads size matters.

Building a basic bomb is not difficult. But building a small warhead - what the nuclear weapons experts term miniaturisation - is a task of a totally different order.

Small warheads enable more than one bomb to be placed on a single missile. They also enable more effective warheads to be placed on lighter mobile missiles.

Both Russia and the United States have deployed such technology. But China has not and is struggling to catch up.

If Beijing really has obtained the plans for one of the most recent US weapons - the W-88 - it will have gained more than a decade in its quest to deploy sophisticated multi-warhead missiles.

The possibility that a Los Alamos computer expert could have supplied Beijing with the details is thus a security lapse of major proportions.

Clinton foreign policy blow

The story is also likely to have an explosive impact upon the Clinton Administration's efforts to defend its policy of engagement with China.

Several weeks ago a Congressional report raised fears about Chinese efforts to obtain US technology for its weapons programmes.

Two US aerospace companies who were launching commercial satellites on board Chinese rockets have been criticised for providing Beijing with assistance that could be used to improve its ballistic missiles.

There is a clear pattern of Chinese espionage and technology smuggling and few Washington insiders are impressed with the Beijing Government's staunch denials.