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Media caption“He was doing strange hand movements, looking up to the sky”: What we know in so far

A man who is critically ill after being exposed to an unknown substance in Wiltshire is a Russian national convicted of snooping for Britain, the BBC understands.

Sergei Skripal, 66, was awarded refuge in the UK following a “spy swap” between the US and Russia in 2010.

He and a woman, 33, were found unconscious on a bench at a shopping centre in Salisbury on Sunday.

Zizzi restaurant in Salisbury has been closed by police “as a precaution”.

The substance has not been identified, but Public Health England said there was no known risk to the public's health.

Wiltshire Police are investigating whether a crime has been committed. They said the pair had no visible traumata but had been seen unconscious at the Maltings shopping centre.

They have declared a “major incident” and multiple agencies are investigating. They said it had not been proclaimed as a counter-terrorism incident, but they were keeping an “open mind”.

Col Skripal, who is a retired Russian military intelligence officer, was jailed for 13 years by Russia in 2006 for spying for Britain.

Sergei Skripal: Who is the former Russian colonel ? Media captionWitness: “They looked like they'd been taking something quite strong” Media captionTemp Asst Chief Constable Craig Holden: “We are unable to ascertain whether or not a crime has taken place” Image caption Public Health England has not confirmed what the substance was Image caption The hospital's A& E was shut on Monday while two people were treated