Video report by ITV News Security Editor Rohit Kachroo
Who is Sergei Skripal?
Sergei Skripal is a 66-year-old Russian ex-spy who was accused of working for MI6 and disclosing the names of Russian agents working in Europe.
In 2010, one the biggest spy swaps since the Cold War took place between Russia and the US, granting Skripal refuge in the UK.
He settled in Salisbury and neighbours have described him as a "quiet" man.
He is now in 'critical condition' after he and another woman were found unconscious in a shopping centre in Salisbury on Sunday.
They are "currently being treated for suspected exposure to an unknown substance," according to Wiltshire police and counter-terror police are helping with the investigation.
Why is this important?
His condition has triggered comparisons with the death of another former Russian spy, Alexander Litvinenko.
A public enquiry in 2016 found that Litvinenko's death had probably been signed off by the Russian president.
His widow, Marina Litvinenko told BBC Radio 4's World Tonight: "It's like deja vu, (like) what happened to me 11 years ago."
She added: "In Russia it is still (an) old-fashioned and old-style KGB system... It's still all the same. If there is an order to kill somebody it will happen."
Sergei Skripal's career in brief:
Skripal retired from Russian militaryintelligence, GRU, in 1999.
However, he admitted working for British intelligence since 1995 after being arrested in Moscow in 2004.
He was reportedly paid more than $100,000 for providing information about GRU agents in Europe.
Convicted for “high treason inthe form of espionage” he was sentenced to 13 years in a high security prison in Moscow.
The Russian president later pardoned Skripal and three other spies in 2010 in exchange for 10 "sleeper agents" planted by Moscow in the US.
The spy swap took place in Vienna's airport.
What happens next?
The police have not yet declared this a "counter terrorism (CT) incident" and are yet to confirm any form of criminal activity.
Former Foreign Secretary Sir Malcolm Rifkind told BBC Radio 4's The World Tonight: "It could indeed potentially have been the FSB (Russian intelligence services) or the Kremlin could have been behind it.
"It could have been some form of criminal response for other reasons, or it could be some form of personal grievance some individual had against these two people or either of them."