- Video report by ITV News Correspondent Geraint Vincent
The Metropolitan Police says a nerve agent was used in an attempt to murder former Russian spy Sergei Skripal and his daughter Yulia, who were found slumped on a bench in Salisbury on Sunday.
These chemical weapons have been used to deadly effect in assassinations and conflicts in the past.
Recent cases include the assassination of North Korea leader Kim Jong Un's half-brother in Malaysia in 2017, and the suspected use of Sarin gas in the ongoing Syrian civil war.
- What is a nerve agent?
A nerve agent is a toxic substance that disrupts signals in the nerves, causing debilitating side effects which can be fatal. Nerve agents are a liquid, rather than a gas, and can seep through the skin.
They were first discovered by accident in the 1930s, when scientists were attempting to find a more cost-effective pesticide.
It proved to be incredibly toxic and posed a risk to mammals, leading it into the hands of the German military, which crafted a weapon of war.
Russia came across such chemical agents for the first time when they swept into East Germany following the Second World War and took control of the plants where they were made.
In the UK, the ingredients required to create a nerve agent are carefully regulated.
Scientists can access some of the components for perfectly legitimate purposes, but will have to explain what their intentions are with it.
Overseas regulation can be less stringent.
- How do they work?
The toxins interfere with the central nervous system, causing the body to become overstimulated.
If you have ever sprayed insect repellent at a fly, you might have seen it drop to the ground and lie on its back, legs twitching.
This is the result of nerve agents taking hold.
Dr Simon Cotton from the University of Birmingham explains: "Our bodies use a molecule called acetylcholine that migrates the gaps between cells - it goes from one cell and slots into the second and triggers a nerve impulse.
"The body has to get rid of acetylcholine that is docked in the receptor because it builds up and you keep getting nerve impulses and become overstimulated.
"Our bodies have got an enzyme that breaks up acetylcholine called acetylcholinesterase - what a nerve agent does is bind to the acetylcholinesterase and stops it from working."
Twitching, spasms, heart failure and respiratory arrest are among the more common side effects.
- How are they used as a weapon?
Different forms of it have evolved, including Sarin, VX and Tabun, all of which have very similar structures and appear to work in the same way.
Only tiny amounts are required for it to take effect - and it is so toxic that it would usually be transported in something tightly sealed and those who apply it would usually need protective clothing.
Doses might be turned into an aerosol can spray, for example.
In an attack on the Tokyo subway which left 12 dead in 1995, liquid sarin was placed in plastic bags which were pierced by umbrellas with sharpened tips.
When Kim Jong Nam - North Korea leader Kim Jong Un's half-brother - was killed at Kuala Lumpur airport last year, a cloth doused with VX was smeared on his face.
- Can victims be treated?
Antidotes do exist, including a medication called atropine - which works by blocking the receptor that acetylcholine usually binds to.
However it is vital that victims receive treatment quickly.