Former Russian spy Sergei Skripal and his daughter may have been poisoned by the deadliest nerve agent ever produced, it has been claimed.
Sergei Skripal, 66, and his daughter Yulia remain seriously ill after the shocking attempted assassination in Salisbury, Wilts., last Sunday.
Police confirmed a nerve agent was used in the attack that also left Detective Sergeant Nick Bailey in hospital.
It is believed to be the first time a chemical weapon has been used on the streets of Britain.
Home Secretary Amber Rudd said the nerve agent is "very rare" and experts appear to have ruled out both VX, developed by Britain in the 1950s and Sarin, developed by Germany in the 1930s.
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Colonel Hamish de Bretton-Gordon, the former head of Britain’s Chemical, Biological Radiation and Nuclear regiment, said that leaves Novichok as the most likely option.
This group of nerve agents was developed by the Soviet Union in the 1970s and 80s and is said to be up to ten times stronger than VX.
Novichoks - meaning 'newcomer' in Russian - were designed as "binary weapons", meaning they are comprised of two relatively harmless ingredients that only become deadly when mixed together.
This makes them easier to transport, handle and gives them a much longer shelf life than other nerve agents.
Colonel de Bretton-Gordon told the Express: "It is designed to be undetectable for any standard chemical security testing.
"Skripal would only have needed to touch it, as he opened a parcel, for it to be absorbed into his bloodstream."
Police and intelligence agencies are continuing to piece together how and when the nerve agent was administered.
So far it had been assumed Mr Skripal and daughter Yulia were exposed to the toxic substance during their visits to a Zizzi restaurant or a nearby pub on Sunday afternoon.
They were both found unconscious on a bench a short time later.
But it has since emerged that Detective Bailey also visited the Skripal home after tending to the victims before he fell ill.
A source told The Times: "He was at both places.
"First he was where they collapsed, trying to help them, then he went to the house, in that order."
Doctors and other staff who tended to the Skripals in the open air have not been affected.Read More
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It is now considered more likely that the nerve gas poisoning took place at Mr Skripal's home just a few miles away from the scene.
It has been reported that Yulia had arrived in the UK from Russia a week before the attack and had brought her father a "gift".
It raises the possibility she may have unwittingly delivered the deadly poison to her father.
The four-bedroom home remains swamped in police activity, with forensic tents covering the property.Read More
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Police turned their attention on Friday to the cemetery where the 66-year-old Russian's wife and son were laid to rest.
Officers in hazmat suits were seen placing a blue forensic tent over his son's memorial stone before appearing to stuff items in several yellow barrels.
The grave of Mr Skripal's wife Liudmila, who was buried in 2012, and the memorial stone of his son Alexander, who was cremated last year, were cordoned off at the London Road cemetery.
Apparent fears of chemical contamination have also seen Mr Skripal's home cordoned off while detectives attempt to pin down the origins of the substance used to incapacitate him.
A short distance away, a convoy of military vehicles rolled into the car park at Salisbury District Hospital to recover a police car.
Around 180 troops, including Royal Marines, RAF Regiment troops and chemical warfare specialists, are understood to have been deployed after Scotland Yard requested specialist help.
Mr Skripal and Yulia, 33, are still in a "very serious" condition a week after they were discovered slumped on a bench in the city centre.
Suspicion is mounting that Russia carried out the attempt on their lives as an act of revenge against the former intelligence officer, who was convicted in 2006 of selling state secrets to MI6.
He was later released as part of a spy swap with the US.
The Kremlin denies responsibility and British ministers have urged caution over apportioning blame until the facts become clear.
Defence minister Tobias Ellwood told the Press Association that the military's presence reflected the "seriousness" of the situation, adding: "We mustn't get ahead of ourselves but we must have a robust response and it's something that we'll be discussing with our Nato partners and with the forthcoming summit in Brussels in July.
"Some big questions arise as to how do you stand up to a clandestine and sinister attack deliberately done to play havoc in our society?"
Mrs Rudd had earlier visited Salisbury and the hospital where DS Bailey is receiving treatment.
Lord Blair, a former Met Commissioner, suggested on the BBC's Today programme the seriously ill detective had visited Mr Skripal's home.
He said: "There obviously are some indications the officer, and I'm very sorry that he has been injured, has actually been to the house, whereas there was a doctor who looked after the patients in the open who hasn't been affected at all.
"So there may be some clues floating around in here."
Police said 21 people had been seen for medical treatment since the incident.
The figure includes members of the public and emergency staff, some of whom have had blood tests as well as receiving support and advice.
The attack is being treated as attempted murder.