Britain's Prime Minister Theresa May makes a speech London

London - It is "highly likely" that Russia was behind the poisoning of a former Russian spy and his daughter in Britain, Prime Minister Theresa May said on Monday.

Experts have confirmed that the poison used against Sergei Skripal and his daughter was a "military-grade nerve agent developed by Russia," May told parliament.

May said her government had summoned the Russian ambassador to London and demanded an explanation for the presence of the nerve agent, promising to discuss with parliament potential action against Russia "should there be no credible response."

May said there were only two possible reasons for the detection of the Novichok nerve agent: that it was used in "a direct action by the Russian state" or because of Russia "losing control" of its stocks of the poison.

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