A major incident has been declared in Wiltshire after it was suspected two people might have been exposed to an unknown substance in Amesbury.
The man and woman, both in their 40s, are in a critical condition at Salisbury District Hospital, Wiltshire Police said.
The force said it was not clear if a crime had been committed, although a number of scenes in the Amesbury and Salisbury area have been cordoned off as a precaution.
The pair were found unconscious at an address in Muggleton Road on Saturday evening and it was initially believed that they may have taken illegal drugs.
Counter-terrorism teams from Scotland Yard were called in to lead the investigation when Wiltshire Police was faced with the poisoning of former Russian spy Sergei Skripal and his daughter Yulia.
On Wednesday morning Scotland Yard was referring media calls to Wiltshire Police.
On Muggleton Road, where the man and woman were found, there was one police officer on duty outside a block of newly-built flats.
The road is on an estate which is still under construction.
Around two miles away there was another police cordon at Amesbury Baptist Centre, also guarded by one police officer.
Breaking: Scotland Yard says man and woman in Amesbury were exposed to nerve agent Novichok, same poison as was used on Sergei and Yulia Skripal in Salisbury— Thomas Daigle (@thomasdaigle) July 4, 2018
It is believed one of the last places the couple were seen in public was a family fun day at Amesbury Baptist Church on Saturday afternoon.
Roy Collins, church secretary, said: "Last weekend we held a community fundraiser and we understand this may well be the last event this couple went to in public."
He said he woke to find the church cordoned off by police at 6am on Wednesday.
"We are all quite puzzled and shocked - naturally the connection with Salisbury and recent events there mean there is a heightened public interest," he added.
"We are praying for the couple, one of our members knows them and clearly there are concerns for them and any others in the community.
"They are not church members or regulars."
Police have confirmed that a man and a woman from Amesbury, who are in a critical condition, have been exposed to the nerve agent Novichok.
It is the same substance used in the attack and poisoning of former Russian spy Sergei Skripal and his daughter Yulia in March.
– What is Novichok?
A group of nerve agents which are more potent and lethal than VX or sarin.
They are made of two separate non-toxic substances that work as a nerve agent when brought together.
Dr Andrea Sella, professor of inorganic chemistry at University College London, said: “Novichok is not really very different from all the classics, you’ve got the same basic chemical framework at the heart of it.
“I’m not sure it’s ever really been used. There’s not much experience of seeing these things, they would have recognised it was some sort of nerve agent, which is part of the reason for the delay [in identifying it].”
– Why was it created?
Novichok, which means newcomer in Russian, was developed by the Soviet Union in the 1970s as a new kind of chemical weapon that would be harder to detect, more potent than existing nerve agents and exempt from the Chemical Weapons Treaty.
Dr Sella 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.”
– How does it work?
Novichok and other nerve agents attack the nervous system and stop chemical messages getting around the body.
They cause the heart to slow down and airways to become constricted, leading to suffocation or brain damage.
“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.”
– What are the symptoms?
Nerve agents, including Novichok, can be inhaled as a fine powder, absorbed through the skin or ingested.
Symptoms can start within seconds or minutes of being exposed and include convulsions, paralysis, respiratory failure and death.
– How can it be treated?
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 not known if there is an antidote available for Novichok.