The substance used in the attack on ex-spy Sergei Skripal and his daughter Yulia has been identified as a kind of nerve agent called Novichok.
What is Novichok?
A group of nerve agents that are more potent and lethal than VX or sarin.
They are made of two separate non-toxic substances that work as a nerve agent when brought together.
Dr Andrea Sella, professor of inorganic chemistry at University College London, said: "Novichok is not really very different from all the classics. You've got the same basic chemical framework at the heart of it.
"I'm not sure it's ever really been used. There's not much experience of seeing these things, they would have recognised it was some sort of nerve agent, which is part of the reason for the delay (in identifying it)."
Why was it created?
Novichok, which means newcomer in Russian, was developed by the Soviet Union in the 1970s as a new kind of chemical weapon that would be harder to detect, more potent than existing nerve agents and exempt from the Chemical Weapons Treaty.
Dr Sella said: "Novichok agents are ones that were kept very quiet by the Russians and developed to try to gain advantage against the more conventional things they knew Western governments had.
"If in fact it is one of the handful of Novichok agents then that starts to point very narrowly at Russia."
How does it work?
Novichok and other nerve agents attack the nervous system and stop chemical messages getting around the body.
They cause the heart to slow down and airways to become constricted, leading to suffocation or brain damage.
"It must be excruciatingly painful and unbelievably violent," Dr Sella said.
"You have very painful muscle contractions, vision goes pretty quickly and what little you can see is blurred, then you can't breathe."
What are the symptoms?
Nerve agents, including Novichok, can be inhaled as a fine powder, absorbed through the skin or ingested.
Dr Sella said: "There are various versions and various kinds of formulations, they are typically oily liquids which go through the skin.
"It's possible that someone ingested it and then it takes time for the stuff to detach itself and then get to you."
Symptoms can start within seconds or minutes of being exposed and include convulsions, paralysis, respiratory failure and death.
How can it be treated?
The treatment for nerve agents is to administer an antidote immediately but some of the damage from the chemical and oxygen starvation can be irreparable.
It is not known if there is an antidote available for Novichok.