As the world's chemical weapons watchdog is meeting, at Russia's request, to discuss Britain's allegations that Russia was behind the poisoning of an ex-spy in England last month, here is some background about the Organisation for the Prohibition of Chemical Weapons (OPCW).

When did it start?

Founded in 1997, the OPCW, based in The Hague, oversees the application of The Chemical Weapons Convention (CWC) aimed at ridding the world of toxic arms.

Chemical weapons, such as mustard gas, were first used on the battlefields of World War I, later being used in 1988 by late Iraqi dictator Saddam Hussein against civilians in Halabja, Iraq. 

Other incidents include a subway sarin gas attack in Tokyo by a Japanese cult in 1995.

After almost 20 years of negotiations, the CWC took effect on 29 April, 1997. 

What does it do?

The OPCW now has 192 member nations and has overseen the destruction of 96% of the world's declared chemical weapons stocks of 72,304 metric tons.

Only three nations have not yet signed up to the convention: Egypt, Israel, and North Korea. South Sudan said last year it was moving to join.

The United States is on target to destroy its chemical arms, the last remaining major declared stockpiles, within the next few years.

How does the OPCW work?

A country that has signed the convention must: declare and destroy all chemical weapons it possesses; destroy all such arms abandoned in another country; and destroy any facilities involved in manufacturing such weapons.

The OPCW monitors the destruction of all declared stockpiles and inspects all former sites where chemical weapons were produced, and suspect sites.

It also seeks to verify credible allegations of chemical weapons use, mostly by sending experts, many of them from ex-military or scientific backgrounds, to the site.

Samples are sent to OPCW-selected labs, or to its own laboratory in The Hague  for analysis.

The OPCW has carried out 6,729 inspections at 3,166 chemical weapon-related and 3,563 industrial sites since April 1997. 

What is Russia's role?

Russia signed the CWC in January 1993 and ratified it in December 1997.

In 2013 Russia was instrumental with the US in sealing a deal forcing Syrian President Bashar al-Assad to join the OPCW after Syria had denied for years possessing any chemical weapons.

In September 2017 Russian President Vladimir Putin said Russia had destroyed its last chemical weapons, in a process begun in 2005 and overseen by the OPCW.

But Britain has accused Russia of breaking the convention, saying it manufactured the nerve agent Novichok allegedly used to poison the Russian spy Sergei Skripal and his daughter in England on 4 March.

Russia denies the charge, and Mr Putin has said he hopes the OPCW meeting would put a "full stop" to the allegations.

Are there penalties?

The OPCW is more of a watchdog than a police force and depends on member states being truthful in their declarations about the size and composition of its toxic arms stockpiles.

It is also not mandated to attribute blame or to determine who unleashed such weapons.

Complicating the situation, some chemicals, like chlorine which has been used against civilians in gas form in Syria's civil war, are exempt from any such declarations as they have wide industrial or agricultural uses. 

The CWC contains no specific punitive measures for countries that use chemical weapons. 

To date, despite condemnation of the use of toxic arms in Syria, no state member has been publicly found to have violated the convention.