Porton Down has been developing countermeasures to chemical weapons since the 1950s (Picture: Crown Copyright).
Specialist military teams are to begin dismantling the Salisbury home of former Russian spy Sergei Skripal.
Mr Skripal and his daughter Yulia were contaminated in March 2018 with a substance, later identified as Novichok by the Defence Science and Technology Laboratory (DSTL) in Porton Down.
The military research facility tested for the nerve agent after the attack on the former spy and his daughter and then was used to conduct tests in July 2018 following a second incident in Amesbury.
Why is Porton Down used to test unknown substances?
It is at Porton Down that the majority of British military research into chemical weapons is conducted.
The site was first created in response to the use of chemical weapons in the First World War.
At the time, the chlorine, mustard and phosgene gas used had never been seen before so soldiers had no idea what they were facing.
Now, Porton Down focuses on the defensive study of chemical weapons in order to better protect the British military against any kind of attack - but it's still highly controversial.
As part of its research, the DSTL produces small quantities of chemical and biological agents, which are securely stored and disposed of safely when no longer required.
The Porton Down site also disposes of old chemical weapons when found in the UK.
Dr Michelle Carlin, a toxicologist at Northumbria University, told Forces News after the second nerve agent attack in Wiltshire that the incident was "far more serious" than routine cases, which is why Porton Down was used.
"Levels of personal protective equipment is going to be far greater [at Porton Down] than we would have in a university lab, or an analytical testing lab, or a pharmaceutical lab, because, obviously, the compounds could be far more dangerous," Dr Carlin said.
Following the Salisbury incident, it is likely a process called gas or liquid chromatography was used to test for the nerve agent Novichok, which was used in the attack.
Dr Carlin said: "It would separate out the components and we would use mass spectrometry in order to identify particular irons that were specific to those compounds.
"So we separate and we identify."
After the second nerve agent incident in Wiltshire in 2018, Dr Michelle Carlin told Forces News why Porton Down is used in those types of cases:
One of the reasons that Novichok is so dangerous is that it can be absorbed through the skin or inhaled, meaning that it does not have to be ingested to take its potentially deadly effect.
Nerve agents like Novichok work by interfering with the central nervous system, causing the body to become overstimulated.
In March 2018, Dr Simon Cotton from the University of Birmingham said: "They interfere with the transmitting of nerve impulses.
"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."
Porton Down is responsible for verifying when chemical weapons have been used all over the world, for example when Sarin was deployed in Syria in 2013.