A former Russian double agent convicted of treason in Moscow was critically ill in a British hospital on Tuesday after exposure to an unknown substance, with police saying it was aware of "the fact of state threats."
In its first response, the Kremlin said it was ready to cooperate if Britain asked for help in its investigation of the "tragic situation."
Sergei Skripal, once a colonel in Russia's GRU military intelligence service, was given refuge in Britain after he was exchanged in 2010 for Russian spies caught in the West as part of a Cold War-style spy swap on the tarmac of Vienna airport.
But the 66-year-old former spy 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. British media reported that the woman was Skripal's daughter.
Both were critically ill in intensive care. "It's a very unusual case and the critical thing is to get to the bottom of what's caused these illnesses as quickly as possible," Britain's top counter-terrorism officer said Mark Rowley told BBC radio.
Who is Sergei Skripal?
Skripal was arrested in 2004 by Russia's Federal Security Service (FSB) on suspicion of betraying dozens of Russian agents to British intelligence. He was sentenced to 13 years in prison in 2006 after a secret trial.
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Skripal, who was at the time shown wearing a track suit in a cage in court during the sentencing, had admitted betraying agents to MI6 in return for money, some of it paid into a Spanish bank account, Russian media said at the time.
But he was pardoned in 2010 by then-President Dmitry Medvedev as part of a swap to bring 10 Russian agents held in the United States back to Moscow.
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.
Skripal, though, was cast as a traitor by Moscow. He is thought to have done serious damage to Russian spy networks in Britain and Europe.
The GRU spy service, created in 1918 under revolutionary leader Leon Trotsky, is controlled by the military general staff and reports directly to the president. It has spies spread across the world.
Since finding refuge in Britain, Skripal lived quietly in Salisbury and kept out of the spotlight until he was found unconscious on Sunday.
"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," witness Freya Church told the BBC.
"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."
Another Russian, Alexander Perepilichny, who had been helping a Swiss investigation into a Russian money-laundering scheme, was found dead in 2012 in Britain.
Police ruled out foul play despite suspicions he might have been murdered with a rare poison. An inquest has yet to give a definitive conclusion as to how he died.