A former Russian double agent is critically ill following his suspected exposure to an unknown substance, it is understood.

The BBC reported Sergei Skripal, 66, is in hospital along with a woman in her 30s, believed to be known to him, after they were found unconscious on a bench in a shopping centre in Salisbury, Wiltshire on Sunday.

Skripal was convicted in 2006 of passing state secrets to MI6 before being given refuge in the UK as part of a spy swap.

The former colonel in the Russian military intelligence, who was sentenced to 13 years in prison, was among four other convicts who were given pardons and one of two sent to Britain in 2010 in a deal that was said at the time to be the largest exchange since the Cold War.

Wiltshire Police said officers were as yet "unable to ascertain" whether the pair have been victims of a crime and the force reassured people there is no risk to the wider public.

Temporary Assistant Chief Constable Craig Holden said: "The pair, who we believe are known to each other, did not have any visible injuries and were taken to Salisbury District Hospital.

"They are currently being treated for suspected exposure to an unknown substance. Both are currently in a critical condition in intensive care.

"This has not been declared as a counter terrorism incident and we would urge people not to speculate. However, I must emphasise that we retain an open mind and we will continue to review this position."

Garry Kasparov, a former world chess champion and critic of Vladimir Putin, was among those who drew comparisons with the death of Russian dissident Alexander Litvinenko in 2006.

A spokesman for the Russian Embassy in the UK, when asked for comment on the incident, said: "Neither relatives nor legal representatives of the said person, nor the British authorities have addressed the embassy in this regard."

Salisbury District Hospital declared a major incident but told patients to attend appointments as normal unless advised otherwise.

Officers were called to The Maltings by a member of the public on Sunday shortly after 4pm.

Freya Church, who saw the couple on the bench, told the BBC they "looked so out of it".

She said: "On the bench there was a couple, an older guy and a younger girl. She was sort of leant in on him, it looked like she had passed out maybe.

"He was doing some strange hand movements, looking up to the sky.

"It looked like they had been taking something quite strong."

The force said a number of areas in Salisbury had been cordoned off in relation to their investigation.

A spokesman for Public Health England (PHE) said anyone exposed to the unknown substance had been decontaminated "as is standard practice in situations like this".

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."

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.

The incident comes at a delicate time for UK-Russia relations, with a report from the Commons Foreign Affairs committee last year describing the relationship between the two countries as being at "its most strained point since the end of the Cold War".

In a speech in October, MI5 Director General Andrew Parker said the agency is working "against espionage and other clandestine activity by Russia and other foreign states who seek to do Britain harm".

In evidence to the Commons Intelligence and Security Committee, MI6 described the Russian state as "formidable adversaries".

In a heavily redacted passage of its annual report for 2016-17, the committee, which has access to highly classified material, concluded: "The events of the past decade or so show that the threat from Russia remains significant. The Agencies' focus on Russia must be maintained."

In 2006, Mr Litvinenko died in London after drinking tea laced with radioactive polonium-210.

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

Reuters