Amesbury poisoning: What are Novichok agents and what do they do?
Police have confirmed that a couple who were found unconscious in Wiltshire were exposed to Novichok - the same group of nerve agents that a former Russian spy and his daughter were poisoned with in March.
Officers say there is nothing in the backgrounds of Charlie Rowley, 45, and Dawn Sturgess, 44, to suggest they were deliberately targeted.
On Thursday, Home Secretary Sajid Javid said there was a "strong working assumption" that the couple came into contact with a similar nerve agent in a different location to the sites which had been part of the clean-up operation in near-by Salisbury after the Skripal poisoning.
That attack was blamed on Russia by the UK, though Russia denied involvement.
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
Some variants of Novichok are thought to be five to eight times more toxic than the 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.
3) How long does Novichok last?
Home Secretary Sajid Javid said he could not rule out the possibility that the nerve agents used in the two incidents were from the same batch, and this was one of the main lines of inquiry that scientists would be pursuing.
Experts are divided on the likelihood that the couple came across the same Novichok disposed by whoever administered it in March to the Skripals.
Dr Mirzayanov cast doubt on the theory, saying Novichok would have decomposed in the four months since the Skripal attack.
But Vladimir Uglev, a scientist who claims he invented the Novichok agent used in the Skripals' poisoning, said this was wrong and the substance is "very stable".
Other experts say the chemicals are designed to be persistent and could last for months or years, particularly if they were kept in containers.
"They [Novichok nerve agents] don't evaporate, they don't break up in water," said Andrea Sella, professor of inorganic chemistry at University College London.
One difficulty is that Novichok is less well studied and understood than other nerve agents, and there is no official scientific data on how long they last.
Detailed analysis of the substance will need to take place before experts will know for certain whether this is the same batch of Novichok.
4) 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.
5) 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 or ingestion, though they could also be absorbed through the skin.
Chemical weapons expert Hamish de Bretton-Gordon said it was likely that the Wiltshire couple had somehow ingested Novichok - perhaps by transferring it from skin to the mouth.
6) 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 said 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.
7) What has Russia said?
The state media in Russia are deflecting any suggestions of a Russian link to the new poisoning, calling the UK government's demands for an explanation "a dirty political game".
Moscow previously denied any involvement in the Skripals' poisoning and demanded proof.
Its foreign ministry has insisted there had never been any research conducted on Russian soil "that would bear the direct or even code name of Novichok".
But the UK foreign office has said it has information indicating that "within the last decade, Russia has investigated ways of delivering nerve agents likely for assassination".
The UK 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.
8) Could anyone else have made Novichok agents?
Dr Mirzayanov believes Russia had 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 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 stored 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 said.
Whether this has happened in the UK's chemical defence laboratory is not known.