Former Russian military intelligence officer Sergei Skripal, who was convicted in 2006 of spying for Britain, was critically ill on Monday after exposure to unidentified substance in Britain, a source close to the investigation told Reuters.

Skripal, once a colonel in Russia's GRU military intelligence, was convicted in Russian of betraying agents to British intelligence before he was later swapped as part of a Cold War style spy swap in Vienna in 2010, Reuters said.

Police said two people had been found unconscious on a bench on Sunday in the city of Salisbury. They were being treated for "suspected exposure to an unknown substance" and they remained critically ill, police said.

Read alsoThe Washington Post remembers 10 Putin critics who died violently or in suspicious waysIt was unclear what the substance was.

The two people were a man aged in his 60s and a woman aged in her 30s and did not have any visible injuries, police said in a statement.

"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 statement said.

A hospital in Salisbury said earlier on Monday it was "dealing with a major incident involving a small number of casualties." It said staff and patients should come to the hospital as usual.

Relations between Britain and Russia have been strained since the murder of ex-KGB agent Alexander Litvinenko in London in 2006, a killing which a British inquiry said was probably approved by President Vladimir Putin.

Skripal was sentenced in Russia to a 13-year prison term before he was pardoned in 2010 as part of a spy swap with the United States.

The swap took place on the tarmac of Vienna airport where a Russian and a U.S. jet parked side by side before the agents were exchanged.