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Johnson said Russia "must give answers" after a military grade Novichok nerve agent hospitalised Skripal, his daughter and a British police officer.
"There can be now doubt what was used and there remains no alternative explanation about who was responsible," Johnson said in a statement, after testing by four laboratories affiliated with the global chemical weapons watchdog confirmed British findings on the nerve agent used in last month's attack.
"Only Russia has the means, motive and record."