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.
A number of scenes in the Amesbury and Salisbury area have been cordoned off as a precaution, although the force said it was not yet clear if they were the victims of a crime.
Public Health England (PHE) advised that it did not believe there to be a "significant health risk" to the wider public, although its advice was being continually assessed.
The incident comes after former Russian spy Sergei Skripal and his daughter, Yulia, were left poisoned by a suspected military nerve agent in Salisbury in March.
Police said the man and woman were found unconscious at an address in Muggleton Road on Saturday evening and it was initially believed that they had taken illegal drugs, however further tests were being carried out.
A police statement said: "Wiltshire Police and partners have this evening declared a major incident after it is suspected that two people might have been exposed to an unknown substance in Amesbury.
"Emergency services were called to an address in Muggleton Road, Amesbury on Saturday evening (June 30) after a man and woman, both in their 40s, were found unconscious in a property.
"They are both currently receiving treatment for suspected exposure to an unknown substance at Salisbury District Hospital. They are both in a critical condition.
"It was initially believed that the two patients fell ill after using possibly heroin or crack cocaine from a contaminated batch of 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."
A number of scenes believed to be areas they frequented before they fell ill were cordoned off overnight.
Meanwhile there was an increased police presence in and around Amesbury and Salisbury.
Salisbury District Hospital was "open as usual" and officials advised people to attend routine appointments unless they are contacted to do otherwise.
A PHE spokesman said: "The current advice from PHE England, based upon the number of casualties affected, is that it is not believed that there is a significant health risk to the wider public. This will be continually assessed as further information becomes known".