Who is he?
Skripal is a former Russian colonel who was given a 13-year jail sentence in 2006 after being convicted of spying for Britain.
He helped Britain’s M16 foreign intelligence agency unmask dozens of Russian agents, according to Associated Press.
The 66-year-old was pardoned in 2010 as part of a spy swap with the United States, according to media reports at the time.
It was the biggest exchange in decades: Anna Chapman was one of 10 Russian sleepers arrested in the US and sent back to Russia.
In return came four alleged double agents, including Skripal who was given refuge by Britain.
Who would want to try and kill him?
Relations between Britain and Russia have been strained since the poisoning of ex-KGB agent Alexander Litvinenko in London in 2006, a killing which a British inquiry said was probably approved by President Vladimir Putin.
Analyst Glenmore Trenear-Harvey told NBC News says if Skripal and his daughter have been attacked it’s likely to have been again authorised by Putin.
Moscow says it doesn't know what happened to Skripal but said it stands ready to help British authorities if they ask for assistance.
“One thing Putin has been doing recently, both in his state of the union address and generally bolstering the Russian people is saying, ‘it's revenge’," said Trenear-Harvey.
“And it's a warning to any other dissidents within the security forces that it might take some time but we will get you.”
Skripal was cast as a traitor by Moscow and he is thought to have done serious damage to Russian spy networks in Britain and Europe, reports Reuters.
Trenear-Harvey said it would make sense for Putin to want revenge.
“He wasn't just simply giving away secrets, Skripal gave away to the British intelligence, MI6, the names of Soviet and Russian agents,” he said. “It's a very serious crime.
“He was pardoned by President Medvedev at that time, but in the same way they say that intelligence officers never retire, spies never retire.”
Since emerging from the world of espionage and betrayal, Skripal is said to have lived modestly in Salisbury and kept out of the spotlight until he was found unconscious on Sunday at 1615 CET.
This quiet existence has prompted the question: why bother to attack him now?
Trenear-Harvey said: “It is most peculiar that it should happen now, but I don't consider Skripal to have been a low-grade spy, and in fact if it is tracked back to President Putin, it is a warning to any other current spies operating on behalf of the west.”