LONDON 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, 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 and review cooperation on other issues. It said 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.
Prime Minister Gordon Brown, speaking after meeting German Chancellor Angela Merkel in Berlin, said he wanted good relations with Russia, but also wanted justice to be served. He said he made no apologies for the diplomatic expulsions.
"We want to work together with Russia constructively on all the major international issues that we face," said Brown, mentioning Iran and the Middle East.
But, he added: "A murder did take place. It has been investigated by the independent prosecuting authorities. They have laid a charge and they have made it clear who they want to try for this crime.
"We're sad that the cooperation has not been forthcoming. We have therefore had to take the action that we have taken."
British prosecutors want to charge former Russian state security agent Andrei Lugovoy with the murder of Litvinenko, a British citizen who died 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 of Russian citizens.
Russian President Vladimir Putin has said the demand is politically motivated. Lugovoy asserted his innocence on Monday.
"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."
Both Washington and the European Union are at loggerheads with Moscow over issues ranging from missile defense and energy policies to the future of Serbia's Kosovo province.
Putin's decision at the weekend to suspend participation in a treaty limiting armed forces in Europe has further raised concerns about a new "Cold War".
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 the sentiment, saying while the 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 jeopardize 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."
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.
In London, Alexander Goldfarb, a Russian emigre and close friend of Litvinenko's, expressed pleasure and surprise at Britain's decision to expel diplomats.
"It's a great, and appropriate, response ... I personally did not expect such a strong reaction."
(Additional reporting by Mark Trevelyan in London, Oleg Shchedrov and Michael Stott in Moscow and Madeline Chambers in Berlin)