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Russian President Vladimir Putin had issued a chilling threat to the former double agent who is critically ill in a UK hospital after being poisoned.

Putin said "traitors will kick the bucket" when Sergei Skripal was sent to the UK in a Cold War-style spy swap between Russia and the US, it was reported.

The 66-year-old former colonel was convicted in 2006 of passing state secrets to MI6 and betraying dozens of Russian agents before being granted refuge in the UK.

Skripal and his daughter Yulia, 33, were both exposed to a substance and are fighting for their lives in hospital amid suspicions that it was a state-sponsored assassination attempt orchestrated by Russia.

Whitehall sources have told The Times that the poisoning was being treated as an assassination attempt linked to Russia.

Russia has denied the allegations and challenged British authorities to reveal what they know about the incident which has triggered a diplomatic row and prompted crisis talks in Whitehall.

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The Government is holding a meeting of its emergency committee Cobra on Wednesday morning, to be chaired by Home Secretary Amber Rudd.

Counter-terror officers are investigating after the father and daughter were found slumped on a bench near The Maltings shopping area in Salisbury, Wiltshire, just after 4pm on Sunday.

They had visited The Bishop's Mill pub and a Zizzi restaurant before they were discovered by passers-by in a mystery that has sparked a number of theories and accusations over how the pair ended up in hospital.

A number of scenes in Salisbury have been cordoned off and some have been decontaminated by workers wearing full-body protective suits and masks, but police and Public Health England have assured the public that they are not at risk.

Cordons are now in place at a further scene near Solstice Park in Amesbury, about 15 miles north of Salisbury, as a precaution, said police.

CCTV obtained by police shows a man and woman 'of interest' walking through an alleyway near the bench, shortly before Skripal and Yulia were found.

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A military research facility in Porton Down, which has state-of-the-art equipment to look for trace amounts of substances, is involved in examining what could have caused Skripal and his daughter to fall ill.

Russia's Foreign Ministry has hit back and said allegations that Moscow may be involved were "groundless".

Spokeswoman Maria Zakharova said Britain should investigate the incident before blaming Russia.

Foreign Secretary Boris Johnson has said the UK would respond robustly if it turns out Russia is to blame for the illness that struck down the Skripals in the medieval cathedral city.

Scotland Yard said detectives were "keeping an open mind as to what happened", and that the incident had not been declared a terrorist incident.

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It also emerged that Yulia had criticised Putin on Facebook, commenting on a letter calling him "the worst president in the world", the Telegraph reported.

She wrote “nice” when a friend accused the Russian president of stealing from the poor and deserving jail.

Yulia is understood to have been visiting Britain from her home in Moscow.

The Telegraph also reported Putin issued a threat as Skripal and the other turned spies were released, saying in a televised addressed: "Traitors will kick the bucket. Trust me.

"These people betrayed their friends, their brothers-in-arms."

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Putin added: "Whatever they got in exchange for it, those 30 pieces of silver they were given, they will choke on them."

Skripal, who revealed Russian secrets to British intelligence agents in return for more than £100,000 after being recruited in 1995, was released as part of a spy swap on the tarmac of Vienna airport.

The former colonel in Russia's GRU military intelligence service, who was sentenced to 13 years in prison, was among four convicts who were given pardons by then-President Dmitry Medvedev.

Skripal was one of two former spies sent to Britain in 2010 - with 10 Russian agents held in the US heading back to Moscow - in a deal that was said at the time to be the largest exchange since the Cold War.

The poisoning incident has drawn comparisons with the death of Russian dissident Alexander Litvinenko, who was poisoned with radioactive polonium-210 in London in 2006.

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The incident follows a "long tradition of Russia eliminating its enemies", an expert on Russian politics has said.

Dr Paul Flenley, a senior lecturer in politics and international relations at the University of Portsmouth, said Russia and its secret police had a "code" to avenge treachery.

He said: "It's all speculation at the moment, but there's a long tradition, since the 1920s, of Russia eliminating its enemies.

"The secret police are well trained. There's an assumption you can't escape, there's a code and if you break it, if you betray them, they will get you - it's part of their discipline."

But Dr Flenley said Putin was not necessarily the source of such acts.

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He said: "There's tendency to assume everything is orchestrated by Putin, and of course he likes that, it amplifies his power, but there are other possibilities, there are business groups and mafia-like groups who could be seeking revenge."

Investigators are trying to determine what poisoned Skripal and his daughter.

The Defence Science and Technology Laboratory in Porton Down is said to be involved in conducting tests.

Salisbury MP and city minister John Glen told the BBC: "Fortunately just down the road in my constituency at Porton Down defence, science and technology labs exists and they will have taken the substance and will be trying to evaluate what they can, no doubt."

An unnamed ex-radiation biologist said in a comment issued through the Science Media Centre: "If the outcome is not just something like a couple of dodgy kebabs - again less probable with both individuals being symptomatic at the same time and with considerable rapidity if they had just eaten there - then I would look for a chemical source.

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"Decontamination at the scene would also suggest that possibility. However, we shouldn't totally ignore biological contamination of food or the environment.

"However, the latter would have caused a wider response from PHE and the authorities."

They added that toxicologists would be looking at samples of both fluids and tissues as well as being told the symptoms.

Alastair Hay, Professor Emeritus of environmental toxicology at the University of Leeds, said it could take some time before test results are available.

He said: "Individuals cannot provide unlimited amounts of blood for testing so investigations will be guided by the clinical team.

"Some tests are rapid and some candidates will be looked at quickly.

"But if the cause is more unusual, body fluids will require significant clean-up preparation before they can be put in an instrument. So this could take a day or several days."

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Litvinenko's widow said she is glad the death of her husband has "raised awareness about the potential danger posed by Moscow".

Marina Litvinenko wrote in The Times that she had to wait for two-and-a-half weeks for her husband's case to be treated seriously after he was hospitalised and died three weeks after drinking tea laced with radioactive polonium-210 in 2006.

A public inquiry concluded in 2016 that the killing of the outspoken critic of Putin had "probably" been carried out with the approval of the Russian president.

Mrs Litvinenko wrote: "I am ... glad my husband's death created a new awareness in the UK's emergency services and police about the need to react quickly when someone suddenly falls mysteriously ill, as has happened in Salisbury.

"After what happened to us I see that something has changed. The police and medical response in Salisbury started so quickly. There was no waiting. There is instantly huge attention in the media.

"I don't want people to suffer and go through want we did. It is awful. However, I am happy my story has raised awareness about the potential danger posed by Moscow and this could help to save somebody's life."