After arriving in Britain eight years ago as part of an extraordinary "spy swap", Skripal, a former Russian intelligence agent, had been leading a life of quiet anonymity. Anna Chapman, a Russia-born secret agent who had acquired British citizenship but was detained by US authorities, was sent back to Russia along with nine other agents as part of that swap. Anna Chapman, who was deported from the U.S. on charges of espionage, displays a creations by I Love Fashion. Credit:AP Grateful to have been pardoned by the Russian authorities for his decades of espionage, Skripal was enjoying an unexpectedly peaceful retirement. But all that came to an abrupt end on Sunday afternoon, when he collapsed in his adoptive home town on Sunday.

Skripal, a retired army colonel, had been jailed in Moscow in 2006, having been convicted of spying for Britain. He was arrested in 2005 after his cover was blown and charged with 'high treason in the form of espionage'. Skripal was found guilty of passing the identities of Russian secret agents operating throughout Europe, to MI6. He appeared in a high profile trial in Moscow's main military court in August 2006, paraded before the cameras on Russian TV. Prosecutors claimed he had been spying for Britain since the Nineties and had taken tens of thousands of pounds in payments from MI6 agents for information. Skripal was reportedly caught by agents from Russia's Federal Security Service (FSB) passing intelligence to MI6's James Bond-style "spy rock" - a fake stone packed with receiving equipment - in a Moscow park. He pleaded guilty to all the charges and reportedly cooperated fully with the FSB.

He was initially sentenced to 15 years in prison, but the term was later reduced to 13 years, because of his willingness to cooperate. Branded a traitor and a disgrace to his country, Skripal was also stripped of all his military decorations and honours. Despite being in poor health he was sentenced to serve his time in a tough maximum security prison in Moscow. The main entrance of Salisbury District Hospital, England, where a former Russian spy is in critical condition after coming into contact with an "unknown substance." Credit:AP But in 2010, in an extraordinary development, he was pardoned by Dmitry Medvedev, then the Russian president. In a scene straight out of the Cold War, he and three other Western agents, were exchanged for 10 Russian spies being held by the FBI in the US.

His apparent poisoning incident bears similarities to the murder of Alexander Litvinenko, the former Russian agent who was killed by radioactive polonium in a London hotel. "It looks similar to what happened to my husband but we need more information. We need to know the substance. Was it radioactive?," Litvinenko's widow Marina said. Skripal and his female companion were discovered on Sunday afternoon. A witness said she walked past the pair who appeared to have "taken something quite strong". "On the right hand side on the bench there was a couple, an older guy and a younger girl, she was sort of lent in on him. It looked like she had passed out. He was doing some strange hand movements looking up to the sky," Freya Church said. The pair were taken to Salisbury District Hospital where authorities declared a major incident and its emergency unit had to be closed.

Alexander Litvinenko in his hospital bed before his death. Credit:Reuters/File Police wore protective suits to examine the area around the bench where they had collapsed . One well-placed source said a number of police officers who had initially attended the scene had also been treated for possible contamination, although London's Telegraph was unable to verify that. One report suggested a "specialist chemical response unit" had removed an "unknown substance" which had been wrapped in several protective layers. The prospect of a state-sponsored assassination on Skripal was immediately raised by opponents of Vladimir Putin. Garry Kasparov, the former chess world champion and high profile critic of the Russian leader, tweeted: "After the UK's pathetic response to Litvinenko's assassination with polonium in London, why wouldn't Putin do it again?"

Igor Sutyagin, a Russian nuclear expert who had been convicted of spying in 2004, was also sent to the UK with Skripal as part of the spy swap. Now of the Royal United Services Institute in London, Sutyagin said he only knew Skripal for the duration of their flight from Moscow. Asked about Russia's possible involvement, he said: "If everything points to these people, then that's a problem for them." Andrew Foxall, director of the Russia and Eurasia Studies Centre at the Henry Jackson Society, said: "While it is too soon to attribute responsibility, it would be foolhardy if the authorities were not to explore the Russia connection." In a statement, Craig Holden, temporary assistant chief constable of Wiltshire Police, 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 in a critical condition in intensive care."

The incident raises concern that Russian assassins had struck again, 11 years after Litvinenko was murdered. Mark Galeotti, an expert on Russia's secret services, said "the Russians have more animus towards 'traitors' than dissidents." But he said it would be unusual to target an ex-spy who had been jailed, debriefed and exchanged. "One thing that made Alexander Litvinenko a target was that he was still working with the security services." "If there was a belief, rightly or wrongly, that Skripal was working for the security services or did something else to make him a person of interest, it would put him back in the crosshairs."

Early reports suggested that Skripal and the unnamed woman may have been exposed to the synthetic drug, Fentanyl, which is up to 10,000 times more powerful than heroin. Telegraph, London