Russian spy: What are Novichok agents and what do they do?
A former Russian spy and his daughter were poisoned by a chemical that is part of a group of nerve agents known as Novichok, UK Prime Minister Theresa May has said.
Sergei Skripal and his daughter Yulia remain critically ill after the attempted murder in Salisbury on 4 March.
The chemical was identified by experts at the defence and science laboratory at Porton Down.
So what do we know about this group of military-grade nerve agents?
1) They were developed in the Soviet Union
The name Novichok means "newcomer" in Russian, and applies to a group of advanced nerve agents developed by the Soviet Union in the 1970s and 1980s.
They were known as fourth-generation chemical weapons and were developed under a Soviet programme codenamed "Foliant".
According to a senior defector, the Soviets used the plant to produce and test small batches of Novichok. These nerve agents were designed to escape detection by international inspectors.
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2) They are more toxic than other agents
One of the group of chemicals known as Novichok - A-230 - is reportedly 5-8 times more toxic than VX nerve agent.
"This is a more dangerous and sophisticated agent than sarin or VX and is harder to identify," says Professor Gary Stephens, a pharmacology expert at the University of Reading.
A number of variants of A-230 have been manufactured, and one of them was reportedly approved for use by the Russian military as a chemical weapon.
3) Novichok exists in various forms
While some variants of Novichok are liquids, others are thought to exist in solid form.
They can also be dispersed as an ultra-fine powder.
Some of the agents are also reported to be "binary weapons", meaning the nerve agent is typically stored as two less toxic chemical ingredients. When these are mixed, they react to produce the active toxic agent.
This makes the ingredients easier to transport as they only become fully toxic when mixed.
"One of the main reasons these agents are developed is because their component parts are not on the banned list," says Professor Stephens. "It means the chemicals that are mixed to create it are much easier to deliver with no risk to the health of the courier."
4) They can take effect very quickly
If a person inhales a Novichok agent, or even if it touches the skin, it begins to take effect rapidly.
Symptoms can start to show in as little as 30 seconds to 2 minutes.
However, in powder form an agent can take longer to act. Systemic symptoms may not show until 18 hours after exposure.
5) The symptoms are similar to those of other nerve agents
It is thought Novichok agents have similar effects to other nerve agents.
This means they act by blocking the messages from the nerves to the muscles, causing a collapse of many bodily functions.
Symptoms include white eyes, as the pupils become constricted, convulsions, drooling and, in the worse cases, coma, respiratory failure and death.
These agents primarily cause a slowing of the heart and restriction of the airways, leading to death by asphyxiation.
Some variants of Novichok have been specifically designed to resist standard nerve agent antidotes.
If a person is exposed to it, their clothing should be removed and their skin washed with soap and water. Their eyes should be rinsed and they should be given oxygen.