A Russian agent convicted of spying for Britain was fighting for his life amid suspicions he was poisoned in a shopping centre in Wiltshire.
Colonel Sergei Skripal, 66, was in intensive care after being exposed to a mysterious substance as he sat on a bench in the centre of Salisbury. A 33-year-old woman who was with him, is also in critical condition. Both had collapsed and were unconscious when they were discovered.
Reports suggest Skripal had recently gone to police claiming he was fearing for his life.
The incident comes a little over a decade after the murder of Alexander Litvinenko, the former Russian agent who was poisoned by radioactive polonium in a London hotel.
Skripal, a former Russian intelligence agent with the FSB, was jailed in Moscow for spying for Britain but had arrived in the UK in 2010 as part of a prisoner exchange.
Litvinenko's widow Marina Litvinenko said: "It looks similar to what happened to my husband but we need more information. We need to know the substance. Was it radioactive?"
Early reports suggested that Skripal and the unnamed woman may have been exposed to the synthetic drug, Fentanyl, which is up to 10,000 times more powerful than heroin and has been linked to scores of deaths in the UK. Authorities declined to speculate as investigations continue.
An eyewitness told how she saw the pair seemingly "frozen" in place.
Georgia Pridham, 25, had been for a hen's do lunch and was walking back to her friend's car when she saw the couple slumped on a bench.
She said: "He was quite smartly dressed, which caught my eye. He had his palms up to the sky as if he was shrugging and was staring at the building in front of him.
"He had a woman sat next to him on the bench who was slumped on his shoulder. She looked grey and had her hood up.
"I thought 'that is quite odd, they must be on something'. He was staring dead straight. He was conscious but it was like he was frozen and he was slightly rocking back and forward.
"She was just slumped onto his shoulder. I thought maybe she was asleep or passed out."
The couple were unconscious when they were rushed to Salisbury District Hospital. Authorities later declared a major incident and its Accident & Emergency unit had to be closed.
Police said Zizzi restaurant, which was close to where Skripal and the woman were found, was closed in connection with the incident following consultation with Public Health England, suggesting one or both of them may have dined there beforehand. Police wearing protective suits were examining the area around the bench where the couple had collapsed. One source told the Telegraph a number of police officers who had initially attended the scene had also been treated for possible contamination although that was unconfirmed. One report suggested a 'specialist chemical response unit' had removed an 'unknown substance' which had been wrapped in several protective layers.
The prospect of a state-sponsored assassination on Skripal was immediately raised by opponents of Russian President Vladimir Putin.
Garry Kasparov, the former chess world champion and critic of the Russian leader, tweeted: "After the UK's pathetic response to Litvinenko's assassination with polonium in London, why wouldn't Putin do it again?"
Dr Andrew Foxall, Director of the Russia and Eurasia Studies Centre at the Henry Jackson Society, said: "While it is too soon to attribute responsibility, it would be foolhardy if the authorities were not to explore the Russia connection in relation to Mr Skripal's illness."
Mark Galeotti, an expert on the Russian secret services, said: "The Russians have more animus towards 'traitors' than dissidents, as it were." But he said it would be unusual to target an ex-spy who had been jailed, debriefed and exchanged.
"One thing that made Alexander Litvinenko a target was that he was still working with the security services here and with others. If there was a belief, rightly or wrongly, that Skripal was working for the security services, or had done something else to make him a person of interest, it would put him back in the cross hairs."
Agent with a starring role in major spy scandals
After arriving in Britain eight years ago as part of an extraordinary 'spy swap', former Russian intelligence agent, Sergei Skripal had been leading a life of quiet anonymity in the Wiltshire city of Salisbury.
Grateful to have been pardoned by the Russian authorities for his decades of espionage, the 66-year-old was enjoying an unexpectedly peaceful retirement.
Neighbour James Puttock, 47, said Skripal had invited the whole road to a housewarming party when he first moved to the Salisbury area.
He said: "He didn't look like a spy. It's hard to remember anything special about him."
Skripal, a retired army colonel, had been jailed in Moscow in 2006, having been convicted of spying for Britain. He was arrested in 2005 after his cover was blown and was subsequently charged with 'high treason in the form of espionage'. Skripal was found guilty of passing the identities of Russian secret agents operating throughout Europe, to the Secret Intelligence Service (MI6).
Skripal was reportedly caught by FSB agents passing intelligence to MI6's James Bond-style 'spy rock' - a fake stone packed with receiving equipment - in a Moscow park.
He appeared in a high profile trial in Moscow's main military court in August 2006, paraded before the cameras on Russian TV.
Prosecutors claimed he had been spying for Britain since the 1990s and had taken tens of thousands of pounds in payments from MI6 agents for information. He was nicknamed 'the spy with the Louis Vuitton bag'.
Skripal pleaded guilty to all the charges and reportedly cooperated fully with the Russian security agency the FSB. He was initially sentenced to 15-years, but the term was later reduced to 13 years, because of his willingness to cooperate. Branded a traitor and a disgrace to his country, Skripal was also stripped of all his military decorations and honours.
But in 2010, in an extraordinary development, he was pardoned by the then Russian President, Dmitry Medvedev.
In a scene straight out of the Cold War, he and three other Western agents, were exchanged for 10 Russian spies being held by the FBI.
The other three spies released to the West, included nuclear expert, Igor Sutyagin, and former KGB colonel, Alexander Zaporozhsky.
Sutyagin had been arrested in 1999 and convicted in 2004 of spying, with the Russian authorities claiming he had provided information about nuclear submarines to the British and Americans. However he always maintained his innocence, claiming later he had been forced to sign a confession.
Zaporozhsky's activities were believed to have led to the unmasking of Robery Hanssen and Aldrich Ames, two of Moscow's most important spies in the US.
Among those returned to Moscow as part of the deal, was Anna Chapman, the secret agent, who had previously lived in the UK.
The glamorous Chapman moved to London in 2001 after meeting Briton, Alex Chapman, at a party. After a whirlwind romance, the pair married and she spent five years living in the UK, before moving to New York City.
She was arrested by the American authorities in 2010 and charged with spying offences.
Later that year she was deported back to Moscow as part of the spy swap - which was the biggest to take place since the Cold War ended.
The extraordinary exchange took place at Austria's Vienna Airport in July 2010.
What is fentanyl?
Fentanyl is a powerful synthetic opioid analgesic that is similar to morphine but is 50 to 100 times more potent. It is typically used to treat patients with severe pain or to manage pain after surgery.
How do people use it: When prescribed by a physician, fentanyl is often administered via injection, transdermal patch, or in lozenges.
What does it do: Like heroin, morphine, and other opioid drugs, fentanyl works by binding to the body's opioid receptors, which are found in areas of the brain that control pain and emotions. It can drive up dopamine levels in the brain's reward areas, producing a state of euphoria and relaxation.
Why is it dangerous: Opioid receptors are also found in the areas of the brain that control breathing rate. High doses of opioids can cause breathing to stop completely, which can lead to death.