Police in Wiltshire, in the south of England about 10 miles from Salisbury, said that they declared a major incident late on Tuesday after suspicion that two people might have been exposed to an "unknown substance" in the town of Amesbury.
A man and a woman, who were found unconscious in a property on Saturday evening, are both in a critical condition and are receiving treatment for suspected exposure to an unknown substance at Salisbury District Hospital, Wiltshire police said in a statement posted on the department's website.
A number of scenes in Amesbury and Salisbury known to have been frequented by the couple had been cordoned off.
Officers initially thought the collapse was drugs-related but have since said they are "open-minded" as to the cause
"It was initially believed that the two patients fell ill after using possibly heroin or crack cocaine from a contaminated batch of drugs," the force said.
"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," Wiltshire police said in a statement.
Public Health England (PHE) said it did not believe there was a "significant health risk" to the wider public, although its advice was being continually assessed.
A former Russian double-agent, Sergei Skripal, and his daughter were poisoned with a nerve agent in the city of Salisbury in March.