Explainer: Who is the former Russian spy critically ill in London and what happened to him
A former Russian double agent is critically ill in hospital in London after exposure to an unknown substance. Here is what we know so far about the incident
Who is the man critically ill in hospital?
Sergei Skripal (66) is a former Russian military intelligence officer who was convicted in 2006 in a secret trial of spying for the UK and sentenced to 13 years in jail.
He was given refuge in the UK after he was pardoned in 2010 by then Russian president Dmitry Medvedev and then exchanged for Russian spies caught in the West as part of a Cold War-style spy swap on the tarmac of Vienna airport.
The swap, one of the biggest since the Cold War ended in 1991, took place on the tarmac of Vienna airport where a Russian and a U.S. jet parked side by side before the agents were exchanged.
One of the Russian spies exchanged for Skripal was Anna Chapman. She was one of 10 who tried to blend in to American society in an apparent bid to get close to power brokers and learn secrets. They were arrested by the FBI in 2010.
The returning Russian spies were greeted as heroes in Moscow. Putin, himself a former KGB officer who served in what was then East Germany, sang patriotic songs with them.
According to the Guardian, it is believed that Skripal was given a new identity, a home and a pension on arrival in the UK.
What happened to him?
According to Reuters, Skripal and a 33-year-old woman who was known to him were found unconscious on a bench in a shopping centre on Sunday in the English city of Salisbury after exposure to what police said was an unknown substance.
Both are critically ill.
Witness Freya Church told the BBC: "On the bench there was a couple, an older guy and a younger girl. She was sort of leant-in on him. It looked like she'd passed out maybe. He was doing some strange hand movements, looking up to the sky,"
"They looked so out of it that I thought even if I did step in I wasn't sure how I could help, so I just left them. But it looked like they'd been taking something quite strong."
Police sealed off the area where the former spy was found and a pizza restaurant called Zizzi in the centre of Salisbury. Some investigators wore yellow chemical suits.
What have the police said?
The area around the restaurant remains sealed off this morning while counter-terrorism officers are now assisting the investigation.
Metropolitan Police assistant commissioner Mark Rowley said: "Clearly it's a very unusual case and the critical thing is to get to the bottom of what has caused this incident as quickly as possible.
"As you would expect, the specialist resources that sit within the counter-terrorism network that I coordinate across the country and other partners are working with Wiltshire Police to get to the bottom of that as quickly as possible."
He told BBC Radio 4's Today programme: "If you look back at other cases like (Alexander) Litvinenko, if necessary we will bring that investigation into the counter-terrorism network.
"At the moment the key is, though, to get to the bottom of what caused this."
Mr Rowley told Today: "We are doing all the things you would expect us to do. We are speaking to witnesses, we are taking forensic samples at the scene, we are doing toxicology work.
"That will help us get to an answer, I can't say any more at this stage."
Have people been linking this incident with the death of Litvinenko?
Mr Litvinenko, 43, an ex KGB agent and outspoken critic of Vladimir Putin, fled Russia for Britain six years to the day before he was poisoned in 2006.
He died after drinking green tea laced with the rare and very potent radioactive isotope polonium-210 at London’s Millennium Hotel.
It took some time for British doctors to discern the cause of Mr Litvinenko's illness.
A public inquiry in the UK in 2016 found that Putin 'probably' approved the murder of Mr Litvinenko.
Has there been any comment from Russia on the Skripal incident?
Kremlin spokesman Dmitry Peskov called Mr Skripal's illness a "tragic situation" but added "we don't have any information".
He said no-one had approached them for help in the investigation but said "Moscow is always open to cooperation".
Asked about the link being made in the media between Mr Skripal and the death of Mr Litvinenko, Mr Peskov said: "It didn't take them long."