British media say a former Russian spy is in critical condition on Monday after being exposed to an "unknown substance".
Skripal, 66, and a woman were found unconscious on a bench near a shopping area and were taken to a nearby hospital where they were treated.
A spokesman for Salisbury NHS Foundation Trust said earlier: "We are now dealing with a major incident involving a small number of casualties, with a multi-agency response". She was sort of leant in on him, it looked like she had passed out maybe.
In 2006, Skirpal, a retired military intelligence colonel, was jailed for 13 years after he was convicted of handing over to MI6 the identities of Russian spies working undercover in Europe during the 1990s.
They were taken to Salisbury District Hospital and are being treated for suspected exposure to an unknown substance.
Since finding refuge in Britain, Skripal kept largely out of the spotlight until he was seen making odd movements on a bench in Wiltshire on Sunday.
It was unclear what substance caused Skripal to fall ill, the source, who spoke on condition of anonymity, said.
A woman who saw them slumped on the bench told the BBC they "looked so out of it".
Relations between Britain and Russian Federation have been strained since the murder of ex-KGB agent Alexander Litvinenko in London in 2006 .
Among the Russian agents deported from America as part of the deal was Manhattan socialite and diplomat's daughter Anna Chapman, who was married to a British man and lived in London for several years.
"It looked like they had been taking something quite strong".
A number of locations in the city centre were cordoned off and the A&E department was closed.
He added: "Scientists from PHE's Centre for Radiation, Chemical and Environmental Hazards, will continue to assist the response and review information as it becomes available".
There has been speculation that the incident could be connected to the collapse of two people in Salisbury on Sunday, but this was not confirmed by Salisbury NHS foundation trust or the emergency services.
In 2006, Russian dissident Alexander Litvinenko died after drinking tea laced with radioactive polonium-210.
A British inquiry into the death said Russia's President Vladimir Putin "probably approved" the killing and identified two Russians, Andrei Lugovoi and Dmitri Kovtun, as the prime suspects.
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