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.
France, Germany and the US have backed the UK’s assessment that Russian involvement is the “only plausible explanation”, in spite of Russian denials.
Sergei Skripal, 66, and his daughter Yulia, 33, remain critically ill after the attempted murder in Salisbury on 4 March.
The chemical was identified by experts at the Defence Science and Technology Laboratory (DSTL) 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.
Novichok’s existence was revealed by chemist Dr Vil Mirzayanov in the 1990s, via Russian media. He later defected to the US, where he published the chemical formula in his book, State Secrets.
According to Dr Mirzayanov, the Soviets used the plant to produce and test small batches of Novichok. These nerve agents were designed to escape detection by international inspectors.
2) They are more toxic than other agents
One of the group of chemicals known as Novichoks – A-230 – is reportedly five to eight 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. One of these experimental chemicals – A-232 – was reportedly used by the Russian military as the basis for a chemical weapon known as Novichok-5.
Russia’s ambassador to the UK, Alexander Yakovenko, has suggested British authorities have identified the variant used in the Skripal attack as A-234.
The BBC’s security correspondent Gordon Corera says the implication of these comments is that Russia has been informed by the British of the specific agent used.
But he adds: “So far, British officials have not confirmed that they have communicated this to Moscow, or that the A-234 was the exact agent deployed.
“Based on public sources, A-234 is one of the Novichok family of agents… Little is known about it but the symptoms track closely with those eyewitnesses attributed to Sergei and Yulia Skripal – as do other similar nerve agents.”
3) Novichoks exist in various forms
While some Novichok agents are liquids, others are thought to exist in solid form. This means they could 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 that are easier to transport, handle and store.
When these are mixed, they react to produce the active toxic agent.
“One of the main reasons these agents are developed is because their component parts are not on the banned list,” says Prof Stephens.
4) Some can take effect very quickly
Novichoks were designed to be more toxic than other chemical weapons, so some versions would begin to take effect rapidly – in the order of 30 seconds to two minutes.
The main route of exposure is likely to be through inhalation, though they could also be absorbed through the skin.
However, in powder form an agent might take longer to cause a reaction.
5) The symptoms are similar to those of other nerve agents
Novichok agents have similar effects to other nerve agents – they act by blocking messages from the nerves to the muscles, causing a collapse of many bodily functions.
Dr Mirzayanov told BBC Russian that the first sign to look out for was miosis, the excessive constriction of the pupils.
A larger dose could cause convulsions and interrupted breathing, he said.
“[Then begins the] continuous convulsions and vomiting, and then a fatal outcome.”
Dr Mirzayanov said there were antidotes – atropine and athene – that helped stop the action of the poison, but that they were not a cure.
If a person is exposed to the nerve agent, their clothing should be removed and their skin washed with soap and water. Their eyes should be rinsed and they should be given oxygen.
6) Could anyone else have made Novichok agents?
Moscow has denied any involvement in the Skripals’ poisoning and demanded proof.
Its foreign ministry insists there has never been any research conducted on Russian soil “that would bear the direct or even code name of Novichok”.
The word Novichok, said spokeswoman Maria Zakharova, was an invention of the West when a number of ex-Soviet scientists moved there in the 1990s “taking with them the technologies they were working on”.
But on Sunday, the UK foreign office said it had information indicating that “within the last decade, Russia has investigated ways of delivering nerve agents likely for assassination”.
“Part of this programme has involved producing and stockpiling quantities of novichok.”
The UK has dismissed as “absolute nonsense” Moscow’s allegations that it could have instead produced the toxin itself at the Porton Down research laboratory. The Kremlin has made similar claims about Sweden, Slovakia and the Czech Republic, which have all been denied.
Dr Mirzayanov believes Russia has to have been behind the Skripal poisoning “because Russia is the country that invented it, has the experience, turned it into a weapon… has fully mastered the cycle”.
Russia’s UN ambassador has insisted that development work on Soviet-era nerve agents stopped in 1992, and that existing stockpiles were destroyed in 2017.
But Novichoks were never declared to the OPCW, and the chemicals never formed part of any control regime partly because of uncertainty about their chemical structures, says Prof Alastair Hay at the University of Leeds.
It is quite likely that some government laboratories made minute quantities and storied their characteristics in databases, so that their identity could be confirmed at a later stage if found as an unknown poison in someone’s blood, he adds.
Whether this has happened in the UK’s chemical defence laboratory is not known.
A sample of the nerve agent used in the Salisbury attack is to be given to the OPCW – the independent international body set up to stop chemical warfare – for analysis. It has called the use of the chemicals “extremely worrying” and said those found responsible should be held accountable.
Meanwhile, chemical weapons expert Hamish de Bretton-Gordon has called for the OPCW to be allowed to visit the Russian town where he alleged the nerve agent was made.
“I have it on very good authority that Novichoks were only ever made in Shikhany in central Russia,” he said.