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.
"However, further testing is now ongoing to establish the substance which led to these patients becoming ill and we are keeping an open mind as to the circumstances surrounding this incident," police said.
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.
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."
The UK's head of counter-terrorism policing Neil Basu speaking at a news conference at New Scotland Yard in London after a couple were left in a critical condition when they were exposed to the nerve agent Novichok in Amesbury, Wiltshire.
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."
