Reports from Britain say a man who is critically ill after being exposed to an unknown substance is the alleged former Russian spy Sergei Skripal.
Authorities did not identify the man, but British media identified him on March 5 as Skripal, who was convicted in Russia of passing state secrets to Britain and sentenced to 13 years in prison in 2006.
The retired Russian military intelligence colonel, who is 66, was one of four prisoners released by Moscow as part of a "spy swap" with the United States in 2010.
British police declared a major incident on March 4 after a man in his 60s and a woman in her 30s were found unconscious on a bench at a shopping center in Salisbury, western England.
They were being treated for "suspected exposure to an unknown substance" and they remained critically ill, police said.
Skripal and the woman were said to be in intensive care at a Salisbury hospital.
Britain's National Health Service said it had only limited information about the patients, but there "doesn't appear to be any further immediate risk to public health."
The substance has not been identified, but local media reported that emergency services suspect the powerful synthetic opiate fentanyl may have been involved.
"They looked like they'd been taking something quite strong," the BBC quoted an eyewitness as saying.
"She was sort of leant in on him, it looked like she had passed out maybe," the eyewitness said. "He was doing some strange hand movements, looking up to the sky."
Skripal was arrested in Moscow in December 2004, and convicted by Moscow's military court in August 2006 of "high treason in the form of espionage."
He was found guilty of passing the identities of Russian intelligence agents working undercover in Europe to Britain's Secret Intelligence Service, or MI6, in return for $100,000.
Russia's Federal Security Service (FSB) alleged he had begun working for MI6 while serving in the army in the 1990s.
Skripal pleaded guilty, reports said at the time.
The death of former FSB officer Aleksandr Litvinenko in London in 2006 has strained relations between Britain and Russia.
British investigators have accused Andrei Lugovoi, a member of Russia's lower house of parliament, of carrying out the poisoning of Litvinenko at a hotel in the center of the British capital.
They concluded that Litvinenko ingested the highly radioactive isotope polonium 210 while drinking tea with Lugovoi and his alleged accomplice, Dmitry Kovtun.
Moscow has dismissed the inquiry as "opaque" and "politically motivated."
Kovtun and Lugovoi have denied involvement despite traces of polonium that British investigators say the two left across London.