LONDON — Former Russian spy Sergei Skripal, who was convicted by Russia of betraying agents to British intelligence, was critically ill on Monday after exposure to an unidentified substance in Britain, two sources close to the investigation told Reuters.

British police said two people, a 66-year-old man and a 33-year-old woman, had been found unconscious on a bench in a shopping center on Sunday in the southern English city of Salisbury after exposure to the unknown substance. Both are critically ill in intensive care.

Mr. Skripal, once a colonel in Russia’s GRU military intelligence service, was convicted in Russia of treason in 2006 but exchanged as part of a Cold War-style spy swap in 2010 on the tarmac of Vienna airport. Mr. Skripal is 66 years old.

British police did not release the names of those who were being treated but two sources close to the investigation told Reuters that the critically ill man was Mr. Skripal. It was unclear what the substance was, they said.

“This has not been declared as a counterterrorism incident…,” Wiltshire police’s Temporary Assistant Chief Constable Craig Holden told reporters. “However, I must emphasize that we retain an open mind, and that we continue to review this position.”

Relations between Britain and Russia 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 Kremlin has repeatedly denied involvement in the killing.

Mr. Litvinenko, 43, an outspoken critic of Mr. Putin who fled Russia for Britain six years to the day before he was poisoned, died after drinking green tea laced with the rare and very potent radioactive isotope at London’s Millennium Hotel. — Reuters

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