A man in hospital after he was believed to have been exposed to an unknown substance is a former Russian spy, it is understood.
The BBC reported Sergei Skripal, 66, is critically ill along with a woman, believed to be known to him, after they were found unconscious on a bench in a shopping centre in Salisbury, Wiltshire.
Skripal was convicted of passing state secrets to Britain in 2006 before being given refuge in the UK as part of a spy swap.
The former colonel in the Russian military intelligence, who was sentenced to 13 years in prison in 2006, was among four other convicts who were given pardons and one of two sent to Britain in 2010 in a deal that was said at the time to be the largest spy swap since the Cold War.
Wiltshire Police said because the investigation was in the early stages officers are "unable to ascertain" whether the pair have been victims of a crime and have reassured people there is no risk to the wider public.
A major incident has been declared at Salisbury District Hospital but patients have been advised to attend appointments as normal unless advised otherwise.
Officers were called to The Maltings by a member of the public on Sunday shortly after 4pm.
Temporary Assistant Chief Constable Craig Holden said: "The pair, who we believe are known to each other, did not have any visible injuries and were taken to Salisbury District Hospital.
"They are currently being treated for suspected exposure to an unknown substance. Both are currently in a critical condition in intensive care.
"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."
The force said a number of areas in Salisbury had been cordoned off in relation to their investigation.
A spokesman for Public Health England (PHE) said anyone exposed to the unknown substance had been decontaminated "as is standard practice in situations like this".
He added: "Scientists from PHE's Centre for Radiation, Chemical and Environmental Hazards, will continue to assist the response and review information as it becomes available."