Britain said today it was expelling four Russian diplomats over Moscow's refusal to extradite the main suspect in the murder of Alexander Litvinenko.
"This is a situation the government has not sought and does not welcome. But we have no choice but to address it," British foreign minister David Miliband said in a speech to parliament.
"We have chosen to expel four diplomats, four particular diplomats, in order to send a clear and proportionate signal to the Russian government about the seriousness of this case," he said.
Mr Litvinenko, a former employee of Russia's Federal Security Service, fled to Britain and became a fierce critic of President Vladimir Putin. He died an agonising death in London after ingesting a lethal dose of the radioactive isotope polonium 210.
Relations between London and Moscow have deteriorated sharply since his death.
Moscow has since refused to extradite Andrei Lugovoy, a former state security agent. British prosecutors want to bring him before a British court and charge him with the murder of Litvinenko.
Moscow has dismissed as ridiculous the former KGB agent's deathbed accusation that Putin ordered his killing.
Besides throwing out four diplomats, Mr Miliband said Britain would review the extent of its co-operation with Russia on a number of issues, including changing the way Russian government officials get visas.