LONDON—U.K. Prime Minister Theresa May said it was highly likely Russia was behind the poisoning of a former spy, summoning the country’s ambassador for an explanation and threatening retaliation for what she called an “indiscriminate and reckless act.”

Mrs. May told Parliament on Monday that Sergei Skripal and his daughter Yulia were poisoned last week with a military-grade nerve agent of the Novichok class, a deadly chemical weapon developed in the Soviet Union in the 1970s and ’80s.

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