Police have confirmed that a man and a woman from Amesbury, Wiltshire, who are currently in a critical condition, have been exposed to the nerve agent Novichok.
Scotland Yard are investigating whether the substance that poisoned Charlie Rowley and Dawn Sturgess was from the same batch that was used in the "attempted murder" of former Russian spy Sergei Skripal and his daughter Yulia in Salisbury in March.
Novichok, which is the name for a family of nerve agents, was developed by the Soviet Union in the 1970s and 1980s.
It was said to be the world's most powerful nerve agent.
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.
Novichok is the chemical substance that both a couple in Amesbury and the Skripals were exposed to.
It is a group of nerve agents which are more potent than both VX or sarin, composed of two separate non-toxic substances.
When brought together, they work as a lethal nerve agent.
Novichok, which means newcomer in Russian, was developed by the Soviet Union in the 1970s as a new chemical weapon that would be more potent and harder to detect than existing nerve agents.
No one knew that Novichok existed until 1992, when chemist whistleblower, Vil Mirzayanov, was jailed for revealing that the binary nerve agent had been developed in secret by the Soviets.
It was thought to be eight to 10 times as powerful as anything in America's arsenal at that time.
Dr Andrea Sella, Professor of Inorganic Chemistry at University College London, said: “Novichok agents are ones that were kept very quiet by the Russians and developed to try and gain advantage against the more conventional things they knew Western governments had.”
Russia came across 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 from Novichok and other nerve agents attack and interfere with the central nervous system, which stops chemical messages from getting around the body.
The heart slows down and airways become constricted, leading to suffocation or brain damage.
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.
“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.”
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.
What are the symptoms?
Nerve agents, including Novichok, can be inhaled as a fine powder, absorbed through the skin, inhaled or ingested.
Symptoms can start within seconds or minutes of being exposed and include convulsions, paralysis, respiratory failure and death.
How are they used as a weapon?
Different forms of nerve agents 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.
This latest exposure comes after the Skripals were exposed to Novichok earlier this year.
It is currently unclear whether the two incidents are related.
Dr Sella said it is "very disturbing" that the agent has been detected four months after the first attack, but Novichok is designed not to break down.
"These things are designed to be persistent," he said.
"They don't evaporate, they don't break up in water.
"The last four months have been dry so I suspect they can be there for quite a long time.
"If the substance was sealed, perhaps in a drinks bottle, then it could take even longer to break down", he added.
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.
Can victims be treated?
Antidotes do exist, including a medication called atropine - which works by blocking the receptor that acetylcholine usually binds to.
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 vital that victims receive treatment as soon as possible.
It is not known if there is an antidote available for Novichok.