Britain, the United States, Germany and France jointly called on Russia on Thursday to explain a military-grade nerve toxin attack in England on a former Russian double agent, which they said threatened Western security.
After the first known offensive use of such a weapon on European soil since World War II, Britain has pinned the blame on Moscow and given 23 Russians who it said were spies working under diplomatic cover at the London embassy a week to leave.
Moscow has denied any involvement in the poisoning. Foreign Minister Sergey Lavrov accused London of behaving in a "boorish" way and suggested this was partly due to the problems Britain faces over its planned exit from the European Union next year.