By Washington Correspondent Tom Carver

In March this year Bill Clinton was asked by a reporter whether there was evidence of Chinese espionage during his time as president.

He replied carefully: "If you are asking me whether there has been such evidence, I have not been told of any."

It was a typical Clinton-esque answer, neither one thing nor the other, occupying the grey middle ground of plausible deniability.

When the Ministry of Defence was confronted by my report which suggested that the UK's nuclear deterrent could be compromised by the apparent epidemic of espionage at America's nuclear laboratories, defence minister John Spellar was equally ambiguous.

Spying revelations have forced President Clinton on to the defensive

"We are not aware of any unauthorised release of nuclear warhead information of UK origin, or of any information which could help others detect our Trident nuclear submarines." He did not deny it might have happened.

The truth is that this multi-layered scandal is still unfolding. But a few days after President Clinton gave his reply, the FBI searched Wen Ho Lee's work computer and discovered that the nuclear scientist had downloaded a large portion of the Legacy Codes, the crown jewels of America and Britain's nuclear weapons programme.

Every one I have spoken to about this scandal has told me that the Legacy Codes represent the combined experience of 50 years of nuclear warhead research.

The codes describe, in millions of lines of computer code and equations, the complex physics of how different warheads detonate and explode.

Of course there are many questions still to be answered.

• Did China access all, some or none of the codes?
• Were they corrupted, either intentionally or unintentionally?
• Even if China has acquired them, what use would they be to her?

But none of these questions can erase the central point: that a scientist with one of America's highest security clearances, Q clearance, was able to walk out of the Los Alamos laboratories with a Zip drive full of American and British secrets.

The radar story seems to be an even more extraordinary lapse of security. Peter Lee, another scientist and no relation to Wen Ho Lee, told his bosses in May 1997 that he was going on holiday to China.

Tom Carver: "The aim of the radar in question is to detect the undetectable"

At the time he was working on one of the most secret American-British research projects. Mr Lee was born in China and only a naturalised American, and the administration was already worried about Chinese espionage.

He held an extensive meeting with Chinese scientists and briefed them on the submarine tracking technology. He was only working on one aspect of it, and it is unlikely that he was able to give the full picture.

Drip by drip revelations

But Chinese espionage works on a principle known as a thousand grains of sand. Rather than trying to cultivate the 'mole' in the classic Cold War sense, who might pass on the full blueprints, they cultivate scientists who give them small bits of information, either wittingly or unwittingly, which they patiently piece together with information from other sources.

Nuclear scientists move freely between the classified and unclassified worlds. They go to scientific conferences all over the world lecturing on the declassified parts of their work.

Even scientists from Britain's Atomic Weapons Laboratory do this. It is possible that Peter Lee was naïve rather than unscrupulous and had no intention of betraying his country.