VX is a nerve agent involving organic phosphorous compounds and sulphur.

It is cheap, relatively easy to make and very nasty - far more toxic than the sarin gas that killed 12 people on a Tokyo subway in 1995.

VX works essentially by penetrating the skin and disrupting the transmission of nerve impulses.

At a high concentration, symptoms progress through coughing, increased perspiration, vomiting and death by suffocation.

The agent attacks both the muscles around the lungs and the respiratory part of the central nervous system.

In liquid form, a drop the size of a pinhead is lethal.

VX was invented in the 1950s by the British Defence Department - the most significant addition to the chemical warfare nerve agent family since World War II.

When the UN's weapons inspection and destruction process began in Iraq after the Gulf War, the Iraqis initially denied that they had been working on VX.

Subsequently, they admitted to having carried out research, and finally conceded that nearly four tonnes of the agent had been produced.

But Baghdad has consistently denied having deployed VX in missile warheads - what the Americans call weaponisation.

Now US army laboratory analysts at the Aberdeen Proving Ground in Maryland say swabs from fragments of Iraqi warheads - inspected by UN monitors at Taji - revealed significant elements identifiable as VX components.