A former Russian spy and his daughter were targeted with a nerve agent in Salisbury, police have revealed.
Detectives are investigating the attempted murder of Sergei Skripal and his daughter Yulia, who are fighting for their lives in hospital.
A Wiltshire Police officer, who was one of the first at the scene, is also in a serious condition.
All three victims are in a coma in intensive care, Sky sources say.
The Metropolitan Police, which is leading the investigation, said there was no evidence of a widespread health risk.
Assistant Commissioner Mark Rowley said: "We are now treating this as a major incident involving an attempted murder by the administration of a nerve agent.
"We believe the two people who originally became unwell were targeted specifically."
The Kremlin has denied involvement but Foreign Secretary Boris Johnson said the UK "will respond robustly" if the attack is found to be "the result of hostile activity by another government".
Hundreds of police officers are working on the case, examining thousands of hours of CCTV footage, and they will remain in Salisbury for several days, Mr Rowley said.
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Police have appealed for people who visited Salisbury town centre or the surrounding areas on Sunday afternoon to contact them.
They have also asked members of the public with any relevant footage or images to come forward.
Dame Sally Davies, chief medical officer for England, said there was a "low risk to the public" and the sites Mr Skripal and his daughter visited have been secured.
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Nerve agents, which are chemical weapons, have been used in assassinations and attacks in war zones in recent years.
Kim Jong Un's half-brother Kim Jong Nam was killed at an international airport in Malaysia last year in an attack using a nerve agent known as VX.
Another well-known nerve agent, sarin gas, was used to kill more than 90 people in a rebel-held area in Khan Sheikhoun, Syria, drawing international condemnation of the Bashar al Assad regime.
Mr Skirpal, 66, and his daughter Yulia, 33, were found slumped on a bench at The Maltings shopping centre on Sunday afternoon.
CCTV has emerged of Mr Skripal buying scratchcards, milk and bacon at a shop in Salisbury days before they were found collapsed.
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Home Secretary Amber Rudd chaired a COBRA meeting about the poisoning on Wednesday and is expected to update the House of Commons on Thursday.
Mr Skripal was convicted in Russia of spying for MI6 in 2006, and later given refuge in the UK as part of a spy swap.
Yulia currently works for Pepsico in Moscow, according to her Facebook account, after moving to Britain for a few years in 2010.
The incident is putting more strain on the already tense UK-Russia relationship.
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In a press conference in Russia, Maria Zakharova of Russia's foreign ministry said the link was being made to whip up anti-Russia sentiment.
She would not confirm if Mr Skripal was still a Russian citizen.
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