UK Prime Minister Theresa May has told parliament that a military-grade nerve agent was used in an attack on former Russian double agent Sergei Skripal. The attack has left Skripal, 66, and his daughter Yulia, 33, in a critical but stable condition. A police officer who attended the victims also remains seriously ill. May says the substance used belongs to the Novichok group of nerve agents, but what are Nivochok agents and what do they do?

What do we know of the attack?

The poisoning attack took place on March 4 in Salisbury city center in Wiltshire, UK. Sergei Skripal and his daughter were found slumped on a bench at 4.15 pm. Traces of the nerve agent used were found at a restaurant where the pair had eaten as well as a nearby pub.

Some 500 people who attended those venues have been advised to wash their possessions as a precaution, while 21 were checked for signs of exposure to the nerve agent. Around 180 military personnel were deployed to help with the decontamination of locations in Salisbury.

The identity of the attacker isn't known, but Theresa May has suggested two possibilities: either this was a state-sanctioned attack by Russia, or Russia has allowed Novichok to fall into the hands of a third party. Theresa May has asked for a credible explanation by the end of today.

What is Novichok?

Novichok is the name of a group of nerve agents developed by the Soviet Union in the 1970s and 80s as part of a secret program codenamed Foliant.

The various Novichok agents each have an organophosphorous core, an organic compound which contains phosphorous, and which is often used in pesticides. In fact, it's thought that the development of Novichok was partly carried out under the guise of pesticide development. Most commonly, this core is either a phosphoramidate or a phosphonate and it is sometimes fluorinated to become a monofluorophosphate.

Other phosphonates and biphosphonates have been used in herbicides, plant growth inhibitors and medicines, while sodium monofluorophosphate can be used in toothpaste. The organic compounds added vary enormously, resulting in agents of varying potency.

The agents were developed to be hard to detect, to work against NATO's chemical protective clothing, and to circumvent the controls put in place by the Chemical Weapons Convention. Some variants have reportedly been designed to resist common nerve agent treatments. In Russian, Novichok roughly means "newcomer" or "newbie."

Just how potent are we talking?

Though it's not known what type of Novichok was used in the attack, it's thought that the variants novichok-5 and novichok-7 can be up to 10 times as potent as VX, the toxic synthetic agent thought to have been used in the 2017 murder of Kim Jong-nam. These are deadly substances by any measure.

In 1999 the US Congress allocated US$6 million to help former Soviet republic Uzbekistan deconstruct and decontaminate a major Soviet chemical weapons testing facility involved in the development of Novichok.

What are its effects?

Novichok agents inhibit the enzyme acetylcholinesterase, which is vital in breaking down the neurotransmitter acetylcholine. Neurotransmitters occur naturally in the body, passing signals between cells, helping us to pay attention, use memory and motivate ourselves.

A Novichok agent essentially causes a build-up in neurotransmitter concentration that can cause involuntary muscle spams which can affect both heart and lung functioning and potentially death by heart failure or suffocation.

The accidental exposure of a Russian scientist to a Novichok agent suggests that they can also cause long-term disability, impairing the use of limbs, as well as epilepsy, hepatitis and cirrhosis of the liver. There may also be potential effects on mental health, including depression.

How is it dispersed?

It's thought that Novichok agents are usually dispersed as a very fine powder, but they are also reportedly capable of deployment as a liquid, gas or aerosol, and potentially via a variety of munitions including bombs and missiles, as well as sprays.

How do we know about Novichok?

Two Russian chemists revealed the existence of Novichok agents in Russian newspaper Moskovskiye Novosti in 1992. The agents were as good as confirmed by the Russian authorities when they brought treason charges against one of the chemists.

Has Novichock been used before?

The attack on Sergei Skripal is thought to be the only use of Novichok agents to date. It's thought to have never been used on the battle field, and there's evidence of the use of Novichok in potentially-similar cases. Fingers-crossed it's also the last.