Originally, no member of the Arab League, which met in Cairo yesterday, was expected to sign although diplomatic sources said that many of the Arab countries privately support the Chemical Weapons Convention and might sign up later. However, last night the League's Secretary-General, Esmat Abdel-Maguid, said: 'There are a number of Arab states which will take part in the signing.' He refused to name the countries but Arab delegates said those that would sign were mostly Gulf states.
The Arab states want Israel to join the nuclear non-proliferation treaty and to put its nuclear installations under international supervision.
The chemicals convention is the first to ban an entire category of weapons of mass destruction and to provide effective measures of verification - which is where previous conventions have failed.
According to US officials, verification of the convention will be 'the most intrusive and widespread in the history of arms control'. It will begin in 1995.
Compliance with the convention will be monitored by the International Organisation for the Prohibition of Chemical Weapons (IOPCW), the last in a series of organisations enforcing a web of interlocking arms control treaties, covering nuclear, conventional and now, chemical weapons.
IOPCW will be based in The Hague, which won the honour of hosting the control agency after a close competition with Vienna. The Hague was the site of the first chemical weapons conferences in 1899 and 1907 but, like the Geneva protocol of 1925 which actually banned chemical weapons, there was no mechanism for enforcing the provisions.
The Chemical Weapons Convention was devised by the UN Conference on Disarmament in Geneva and finalised in December, when it was sponsored by 144 nations. The formal signing by an estimated 115 nations begins at 4pm. The major powers will sign first: the US Secretary of State, Lawrence Eagleburger; Russia's Foreign Minister, Andrei Kozyrev; and Britain's Foreign Office Minister, Lynda Chalker.
Unlike nuclear weapons, chemical weapons have been widely used in war, notably in the First World War. Although some military theorists in the inter-war period favoured them as a more 'humane' form of killing, they have since inspired revulsion, notably after Iraq's use of nerve-gas against Kurdish civilians at Halabja in 1988. As a form of 'human insecticide' they could be attractive to ruthless regimes - for example in the former Yugoslavia - killing people without destroying property.
Only three countries - the US, Russia and Iraq - admit to possessing chemical weapons. But Western experts believe about 20 countries have chemical weapons or active programmes to develop them. Not all are identified publicly, but the list includes countries in the Middle East, Asia and Africa, most of whom deny it.
According to the Stockholm International Peace Research Institute the US still holds 31,400 tons and Russia about 40,000 tons (some estimates are much higher). Iraq's stocks are being destroyed, when UN teams can get to them. By December 1992, officials had reported the destruction of 5,000 filled shells, 5,600kg of mustard- gas and over 40,000 litres of other agents.
The first chemical weapons used in the First World War and the 1980-88 Iran-Iraq war were non-lethal tear-gases. But their use quickly led to lethal agents.
Nowadays, there are four main types of chemical weapons: blister agents (vesicants), such as mustard-gas; choking agents (suffocants), such as chlorine and phosgene, widely used in the First World War; blood agents which cause general asphyxia and sudden death by respiratory arrest; and nerve agents (neurotoxins), such as Sarin, Tabun and VX, particularly virulent chemicals that prevent the body switching off the nerves, resulting in muscular contractions and convulsions.
There are also 'psychochemicals' such as BZ, with an effect similar to LSD. They are classified as highly toxic, but are not lethal.