Sergei Skripal has been named as the man who, along with a woman, was found unconscious in a shopping centre in Salisbury.
Police are investigating whether a crime has been committed, following the incident which began at 4.15pm GMT on Sunday (11.15am Monday NZDT) at the Maltings shopping centre in central Salisbury.
Neither police nor the hospital have offered details thus far, but a "major incident" has been declared at the Salisbury District Hospital as a result of the exposure.
But few could avoid invoking the name of Litvinenko - the former Russian agent who died after drinking polonium-210-laced tea in a swanky London hotel in 2006.
Investigators are reviewing his death to see if there are any similarities with the Skripal case. Both are now being treated in hospital, where they are in intensive care.
Sergei, a former Russian army colonel, was convicted of passing the identities of Moscow agents working undercover in Europe to MI6 in the year 2006.
The Salisbury District Hospital, where the pair was taken in a critical condition and treated for exposure to an unknown substance, was forced to close its emergency room.
Freya Church, who saw the couple on the bench, told the BBC they "looked so out of it".
"It looked like they'd been taking something quite strong", she said.
"And while it'd be wrong to prejudge the investigation, I can reassure the House that should evidence emerge that implies state responsibility, then Her Majesty's government will respond appropriately and robustly", he said. She said the woman appeared to be collapsed against the man, who was making odd hand gestures and looking up at the sky.
He passed information classified as state secrets and was paid for the work by MI6.
Mr Skripal was sent to Britain as part of a spy swap in 2010, having been given a pardon from his prison sentence.