Described by neighbours as a quiet man who dressed casually and drove a BMW, Sergei Skripal was once colonel in Russian military intelligence before he was convicted of spying for MI6.
The 66-year-old was accused of working for MI6 over several years, in particular disclosing the names of several dozen Russian agents working in Europe.
He was sentenced to 13 years in a high-security prison in August 2006, before being freed in a 2010 deal which saw 10 Russian sleeper agents expelled from the United States.
Skripal retired from military intelligence, often known by its Russian-language acronym GRU, in 1999, the Associated Press said.
He went on to work at the Foreign Ministry until 2003 before becoming involved in business.
He was arrested in 2004 in Moscow and admitted he was recruited by British intelligence in 1995 and had provided information about GRU agents in Europe, for which he was paid more than 100,000 dollars.
Skripal was one of four agents pardoned and released by Moscow in what was said at the time to be the biggest spy swap since the Cold War.
He was flown to the UK along with another of the men freed by Russia in the exchange - military analyst Igor Sutyagin, who was serving a 14-year sentence for spying for the US.
The spy swap took place on July 9, 2010 on the tarmac at Vienna's airport and a Boeing 767-200 carrying the four agents was understood to have later touched down at RAF Brize Norton in Oxfordshire.
The Foreign Office refused to comment at the time on whether the men would remain in the UK.
But it appears Skripal settled for a quieter life in the medieval cathedral city of Salisbury, Wiltshire, for several years until it emerged he was understood to be critically ill in hospital after what police said was suspected exposure to an unknown substance.
Neighbours told the BBC police arrived at his home at around 5pm on Sunday and have been stationed there since.
They added he was friendly and had lost his wife in recent years, the broadcaster said.
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Neighbour Mark Medhurst, 43, told the Daily Mail that Skripal drove a BMW and kept the lights off at his home.
James Puttock, 47, told the Guardian his neighbour was "very quiet".
He said: "He was always walking past, but he did sometimes drive his BMW 3 Series. He never really looked smart, he looked very casual."
Meanwhile, his compatriot Sutyagin, who always protested his innocence, told the Associated Press in an interview in August 2010 he wanted to return to Russia, saying: "It's my country. I am not on the run."
He went on to become a research fellow at the Royal United Services Institute specialising in US-Russian relations.
Of his conversations with Skripal on the 2010 flight out of Russia, he told the Guardian: "He talked about his family. It seemed to me it was his family which was his major joy."
Among the Russian agents deported from America in the swap deal was Manhattan socialite and diplomat's daughter Anna Chapman, who was married to a British man and lived in London for several years.