It is feared that Mr Skripal – who was convicted in his home country for selling secrets to MI6 – was singled out for a state-sponsored assassination plot. Here's everything you need to know about the ex-spook.
Who is Sergei Skripal?
Sergei Skripal was born on June 23, 1951.
He was a Russian officer who was convicted in 2006 of spying for the United Kingdom.
Skripal was accused by Russian prosecutors of working for MI6 in the late 1990s and passing secrets to the British using a James Bond-style spy rock.
At the time, he was serving in the Russian Ground Forces and was nicknamed 'the Spy with the Louis Vuitton bag'.
What happened to Sergei Skripal and his daughter Yulia?
The ex-KGB spy and his daughter Yulia, 33, were exposed to an "unknown substance" while out in Salisbury on Sunday March 5.
They were found slumped on a bench in a "catatonic state" and anti-terror police are investigating CCTV believed to show the Russian father and daughter before the suspected "poisoning".
Russia's UK embassy have denied that their special services were involved.
Sergei also faced personal tragedies involving his wife, son and brother.
His wife Liudmila died from cancer in 2012 and in the last two years he lost his older brother and his 43-year-old son Alexandr.
His son Alexandr was said to have died from liver failure on holiday in St Persburg last year.
What is the latest diplomatic situation around the Skripal "poisoning"?
Wiltshire county's acting police chief Kier Pritchard said "around 21 people" including the Skripals have received medical treatment as a result of the nerve gas attack.
British Home Secretary Amber Rudd said enormous resources are being devoted to determining who poisoned Skripal and his 33-year-old daughter Yulia.
They were found unconscious on a bench, triggering a police inquiry headed by counter-terrorism detectives.
"The use of a nerve agent on British soil is a brazen and reckless act," Rudd told Parliament. "This was attempted murder in the most cruel and public way."
Rudd said Britain would respond strongly when it is clear who is to blame.
The Russian Embassy in London, which has mocked other British politicians for suggesting Russian involvement, tweeted that it agreed with Rudd: "First evidence then conclusions on Mr Skripal's case. Responsible political approach."
Why was Sergei Skripal arrested and how long was he in prison for?
Skripal was arrested in December 2004.
He was charged with "high treason in the form of espionage" and was found guilty.
The former spy was sentenced to 13 years in prison, after Russia claimed he was paid $100,000 to extract classified information and state secrets.
He pleaded guilty at his trial, but was later one of the four prisoners Moscow swapped in 2010 for spies in the US.
It was reported that Skripal managed to return to Britain, but has been keeping a low profile for the last eight years.
The retired spy Colonel and his a woman believed to be his daughter, Yulia, are both critically ill in a Wiltshire hospital after being found slumped in the Maltings shopping centre in Salisbury.
Police said the pair were exposed to an unidentified substance.
They have cordoned off a Zizzi restaurant nearby "as a precaution" which would suggest that one or both of the victims had dined there.
When was the spy swap with Anna Chapman?
Sergei Skripal was brought to Britain during a high profile spy swap with Anna Chapman.
In 2010, four men including Skripal were released by Moscow, while the US handed over 10 Russian "sleeper" agents as part of the deal – Anna Chapman being one of them.
Skripal and an another a man named Igor Sutyagin were flown into the RAF base in Brize Norton in Oxfordshire on July 9 2010.
Sutyagin was accused by Moscow of providing British authorities with information about a nuclear submarine.
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