Allied commanders believe that the only reason he did not use them during the 1991 Gulf War was the threat of nuclear retaliation. Iraq's Foreign Minister Tareq Aziz has said that this possibility did weigh heavily on Mr Hussein and his generals.

This time, however, the Iraqi leader has much more at stake that just eviction of his troops from Kuwait. He is fighting for his life.

In January he ordered thousands of protective NBC (nuclear, biological, chemical) suits and doses of atropine a key nerve gas antidote for his elite troops, suggesting he is prepared to use chemical and biological weapons against invading allied armies.

According to Dr Wyn Bowen, who was part of UN weapons inspection teams on three missions to Iraq during the late 1990s and is now a lecturer in defence studies at King's College, London, any decision to use such an arsenal ''depends on [Saddam Hussein's] threshold perception''.

If the allies are going in to disarm Iraq, Dr Bowen said, the chance of weapons of mass destruction being used is low. ''But if the aim is regime change as the US wants then it increases the probability of them being used. [If] he's resigned to the fact that he's basically dead ... he might want to launch all kinds of crap against Israel.''

The US and Britain have maintained Iraq is continuing with efforts to acquire nuclear weapons in addition to the chemical and biological devices it is widely believed to have and successfully hidden during the last few months of United Nations' inspections.

Baghdad almost certainly had a fledgling nuclear bomb programme during the 1990s. How far it has progressed since is unclear, but it is hard to hide a nuclear programme. The feats of engineering and chemistry required leave a variety of tell-tale signs visible from the air.

Baghdad's biggest problem would be getting hold of the fissile material (plutonium or enriched uranium) needed to make a nuclear bomb. This would probably have to come either from the black market or a rogue government. An assessment late last year, from the International Institute for Strategic Studies in London, said that Iraq could assemble a nuclear weapon ''within months'' if it got the material from abroad.

But if it had to build facilities to do this itself, then it would take considerably longer.

Nato countries stockpiled huge amounts of biochemical agents during the Cold War, but they were eventually dropped from strategic plans. The main reason for that decision was the unpredictable and inaccurate nature of biochemical weapons. Chemicals and spores blow with the wind, and can contaminate large areas for many years. In the scenarios they envisaged, Nato leaders decided too many of their own forces and innocent civilians would have been at risk.

Iraq has reportedly rebuilt several facilities capable of producing chemical weapons since 1991. Although they are under UN and US surveillance, it is thought that some could easily be converted from industrial and commercial usage, allowing Iraq to restart limited production of chemical weapons agents within days or weeks.

Iraq has an estimated 40 sites containing equipment that could be converted to produce chemical weapons agents and their precursors and four facilities that produced chemical munitions until 1991 and could do so again.

The most potent material in Iraq's alleged arsenal is the nerve agent VX. Just one milligram on the skin can kill by disrupting the transmission of nerve impulses in the body, resulting in convulsions, seizures, blindness and eventually death. Although atropine (part of injection ''cocktail'' of drugs given to troops) can serve as an antidote, VX acts so quickly that victims would have to be injected almost immediately to have a chance of survival.

Along with VX, the US and Britain claim that Baghdad has renewed production of mustard gas, sarin, and cyclosarin. They also allege Iraq has stockpiled large quantities of these agents.

Sarin gained infamy as the chemical used in the 1995 attack on Tokyo's subway by the Aum Shinrikyo (Supreme Truth) sect, which killed 12 people. Sarin works in much the same way as VX, although not as quickly.

The sarin used in the Tokyo attack was poorly made and only 30 per cent pure; the offensive smell of the impurities was enough to alert passengers to the fact that something was amiss. Pure sarin, however, is odourless.

VX was developed by British scientists at the secret Porton Down research base in 1952, and its devastating effects were tested. The British traded VX technology with the US for information on thermonuclear weapons.

Anthrax and botulin toxin top the suspected list of biological agents Iraq may have successfully hidden. In 1996, the UN destroyed a factory designed to make up to 50,000 litres of such materials a year.

Having biochemical agents is one thing but turning them into effective weapons depends on the ability to deliver warheads accurately.

Years of inspections following the 1991 Gulf war accounted for 817 of the 819 Scud rockets Iraq was believed to possess, but by salvaging some components and reproducing others Baghdad may now have between 12 and 25.

In addition, UN inspectors have recently found and at least partly dismantled an arsenal of al-Samoud missiles with a potential range of 30 to 40km greater than the 150km limit imposed by UN resolutions.

Dr Bowen said that when he was part of the inspection team in the 1990s, ''we thought that in a post-sanction environment the regime might build such missiles. And now it has been proved.''

The equipment designed to make the shorter-range rockets is certainly capable of being used to make larger missiles, according to Dr Bowen. Whether Iraq could have kept such devices secret during months of intrusive inspections, though, is the big question. The discovery of the prohibited-range rockets gave added credence to the recent and much-criticised British dossier on Iraq's ability to launch weapons of mass destruction.

''The UK document said they were trying to scale up the al-Samoud and [UN inspectors have] proved that on the ground,'' says Dr Bowen.