The exact effects of a neutron bomb remain unknown but the prospect of its development and use has sparked controversy for years.

Throughout the 1970s and 1980s hawks in the western defence establishment supported it as the "clean" nuclear weapon that would prevent the slide towards mutual assured destruction in the outbreak of war.

But anti-nuclear campaigners consider it one of the most appalling results of the nuclear arms race.

Development of a super weapon

Nuclear weapons differ in how much heat, blast, light, pressure and radiation they produce. By altering the physical structure of the device and the proportion of its explosive components, different effects can be achieved.

Research into the neutron bomb began seriously in the 1970s when military scientists in the United States sought to reduce the amount of blast produced by thermonuclear devices and increase the amount of gamma radiation thrown out.

Some of the lead work in the field is said to have been carried out in a French atmospheric detonation in 1967.

The United States wanted to develop a nuclear weapon that would allow it to wipe out a Soviet army as it invaded Western Europe but leave towns and cities intact.

The neutron bomb, or "enhanced radiation" device with its supposed promise of a "clean" kill was believed to be the strategic answer to a hole in the Cold War arsenal.

Whereas a standard thermonuclear device will destroy buildings in a vast shockwave of heat and pressure, the neutron bomb would detonate above a battlefield with, theoretically, little risk of destroying the surrounding area.

The blast would be confined to a radius of no more than a couple of hundred metres but a massive wave of radiation would knock out tank crews, infantry and other personnel.

Even if the buildings did remain, survivors would soon find their bodies filled with elements such as strontium, ensuring that they eventually die of radiation poisoning.

Cold War product

William Peden of the UK-based Campaign for Nuclear Disarmament, said the neutron bomb was a product of the Cold War age - and predicted that China's acquisition of the technology would destabilise the region.

"The technology behind the neutron bomb is relatively simple in nuclear terms," said Mr Peden. "It's a matter of tweaking an existing nuclear weapon.

"What is even more worrying is this kind of technology getting into the hands of those who seek nuclear power.

"The neutron bomb became a way of legitimising nuclear weapons. The radiation would clear up after a strike and it was regarded as the magic wand of nuclear weapons - you could wave it and all the people would be gone."

First warhead

By the late 1970s American nuclear scientists had developed the W-70 neutron warhead to be used with the Lance tactical missile.

Used over a battlefield, a one-kilotonne neutron bomb would kill or incapacitate people over an area twice as large as the lethal zone of a 10 kilotonne standard nuclear weapon - but with a fifth of the blast.

In 1978 President Jimmy Carter halted neutron device production as concern grew over its effects on the arms race.

But according to the damning Cox report into China's nuclear espionage, Beijing's agents had stolen neutron bomb secrets before the decade was out - the first of many warhead designs targeted by the Chinese.

While the US says it never tested a fully working neutron device, officials believe China tested the technology in 1988.

Mr Carter's successor, Ronald Reagan soon reversed the policy and the W-70 warheads resumed production in 1981.

But does it work?

Only a small number of neutron warheads were produced and Washington never deployed the weapon alongside its other nuclear forces in Europe because of the surrounding political controversy.

W-70 warheads appear to have been completely scrapped as part of the arms reductions of 1993.

Critically, the question remains over whether the technology that China has developed actually works.

Under the terms of the 1996 Comprehensive Test Ban Treaty, of which China is a signatory, no detonation of the weapon would be allowed.

William Peden of CND said: "Whatever has been said about the neutron bomb, its actual effects are known only in theory.