He was convicted in Russian Federation in 2006 of passing state secrets to Britain, before later being given refuge in the United Kingdom as part of a spy swap.
He and a woman, 33, were found unconscious in a shopping center in Salisbury, south-west England, on Sunday.
The priority, the police said, is to identify any substances involved.
The retired Russian secret service colonel had been convicted and jailed for 13 years in 2006 for betraying Russian intelligence agents to Britain's MI6 secret service.
Following his arrest in Moscow in December 2004, Skripal confessed he had been recruited by British special services in 1995 and was feeding them information about Russian agents in Europe, receiving over $100,000 for his services.
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This is considered a hard freeze for agricultural purposes, and it poses a danger to frost/freeze sensitive plants. Cooling Off: Once the cold front moves through we'll see drier and cooler air gradually filter into the Valley.
He was one of four convicts given a pardon, and one of two sent to Britain after a spy swap in July 2010 - described as the largest since the Cold War.
Col Skripal, now believed to be 66, was later flown to Britain. A&E Staff were evacuated to a nearby building to establish a triage centre while the facility was closed for decontamination.
They said he was friendly and in recent years had lost his wife.
Freya Church told the BBC it looked like the two people had taken "something quite strong".
"On the bench there was a couple, an older guy and a younger girl", Ms. "She was sort of leant in on him, it looked like she had passed out maybe", she said.
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I think his own heart is more important than my advice, because my advice is purely from the career point of view. Both England and Scotland are involved in a tug-of-war over luring him into their global set-ups.
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.
Relations between Britain and Russian Federation have been strained since the murder of ex-KGB agent Alexander Litvinenko with radioactive polonium-210 in London in 2006, a killing which a British inquiry said was probably approved by President Vladimir Putin.
"The pair, who we believe are known to each other, did not have any visible injuries".
"At this stage it is not yet clear if a crime has been committed", they said in a Twitter post, adding that they are "carrying out a full investigation & working with partner agencies, to clarify exact circumstances".
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