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Britain has expelled four Russian diplomats from London in retaliation for Moscow's refusal to extradite a suspect in the sensational radiation poisoning of a former KGB spy on British soil. Russia immediately threatened Britain with "the most serious consequences for Russian-British relations," Reuters reports.

The victim, Alexander Litvinenko, was assassinated with radioactive poison last November in London.  "We have chosen to expel four diplomats  in order to send a clear signal to the Russian government about the seriousness of this case," said a British official. The clash comes at a low ebb in relations between Russia and the West. (Read more Alexander Litvinenko stories.)