Britain ordered the expulsion of four Russian diplomats on Monday over Moscow's refusal to hand over the main suspect in the murder of ex-KGB agent Alexander Litvinenko, further escalating tension between Russia and the West.
Russia said the decision was "immoral", would prompt retaliation, and could only "entail the most serious consequences for Russian-British relations," according to a foreign ministry spokesman.
Aside from the expulsions -- the first since 1996 -- Britain said it would make it harder for Russian officials to come to the country, review cooperation on other issues and warned the case could harm Russian ties with the European Union as a whole.
"This is a situation the government has not sought and does not welcome. But we have no choice but to address it," British Foreign Secretary David Miliband told parliament.
British prosecutors want to charge former Russian state security agent Andrei Lugovoy with the murder of Litvinenko, who died an agonising death in a London hospital after ingesting a lethal dose of the rare radioactive isotope polonium 210.
Russia has rejected Britain's request to hand over Lugovoy, saying its constitution does not allow extraditions. Russian President Vladimir Putin has said the demand is politically motivated.
"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," said Miliband, adding he was grateful for strong EU support.
"We will discuss with partners the need for future EU-Russia engagement to take our concerns on this case into account."
Relations between Russia and the West have deteriorated with both Washington and the European Union at loggerheads with Moscow over issues ranging from missile defence and energy policies to the future of Serbia's Kosovo province.
Putin's decision over the weekend to suspend participation in a treaty limiting armed forces in Europe has further raised concerns about a new "Cold Front".
Interfax news agency quoted an "informed source in Moscow" -- a traditional reference to high-level leaks -- as saying the Russian response would not necessarily be tit-for-tat.
"This is not our principle," said the source. "It would bring us back to the days of the Cold War. It's a shame that some have this principle entrenched deep in their minds."
Analysts echoed this sentiment, saying that while the British expulsions would provoke a robust response from Russia, they did not expect the row to escalate too far as it might harm commercial ties.
"I don't think they'll do anything that will jeopardise that," said Derek Averre, Russia specialist at Birmingham University. "There won't be a massive kind of escalation because it's simply not in Moscow's interests."
Miliband stressed that a good relationship between London and Moscow was crucial because of international action on climate change, terrorism, Iran, Kosovo, the Middle East peace process, Sudan and nuclear proliferation, among other things.
"For all these reasons we need a relationship based on trust and mutual respect," Miliband said.
Moscow has dismissed as ridiculous Litvinenko's deathbed accusation that Putin ordered his killing.
Britain has rejected a Russian offer to put Lugovoy on trial at home, saying it doubts Moscow's promises of a fair trial.
(Additional reporting by Mark Trevelyan in London, Oleg Shchedrov and Michael Stott in Moscow)