Sergei Skripal, 66, who is reported to be critically ill, was granted refuge in the U.K. following a spy exchange between the U.S. and Russia in 2010.
A man in his 60s and a woman in her 30s were found unconscious in the Maltings shopping center on Sunday afternoon, according to a police statement.
“The pair […] are currently being treated for suspected exposure to an unknown substance,” and “both are currently in a critical condition in intensive care,” according to a police statement.
Craige Holden from the local police force said: “Because we are still at the very early stages of the investigation, we are unable to ascertain whether or not a crime has taken place.”
“A major incident has been declared today and a multi-agency response has been coordinated,” he said.
“Alongside our partner agencies, we are conducting some extensive enquiries to determine exactly what led to these two people falling unconscious and clarify whether or not any criminal activity has happened.”
Skripal was convicted of "high treason in the form of espionage" by a Moscow military court in 2006. He was sentenced to imprisonment for 13 years.
He is one of four Russians exchanged for 10 “sleeper” agents planted by Moscow in the U.S.
Holden said the incident in Salisbury "has not been declared as a CT (counterterrorism) incident and we would urge people not to speculate.”
"The focus at this moment in time is trying to establish what has caused these people to become critically ill, and we are working with partners to prioritize this diagnosis and ensure that they receive the most appropriate and timely treatment,” he said.
Monday's unexplained incident is reminiscent of the poisoning of former KGB agent, Russian dissident Alexander Litvinenko by a radioactive substance in 2006.
Litvinenko died at the hospital shortly after taking polonium-210 while drinking tea in a central London hotel.
His family believes he was working for British intelligence services at the time and was killed by Russian secret agents.
Former KGB bodyguard Andrei Lugovoi and Dmitry Kovtun have been identified as the prime suspects, however both have denied any involvement.