As the UK expels four Russian diplomats following Moscow's refusal to extradite the main suspect in the murder of former spy Alexander Litvinenko, BBC diplomatic correspondent Jonathan Marcus considers the implications of Britain's move.
There is no doubting the seriousness with which the British government views this case.
A man was murdered in London and the method employed - the use of a highly radioactive substance - raised wider issues of public safety.
So a tough response was expected.
But Britain has had to weigh up its potential actions carefully.
In his statement, Foreign Secretary David Miliband made it clear that Russia remained a key player on the international stage and that Britain wanted good relations with Moscow.
The aim of the measures that Britain had taken, principally the expulsion of four diplomats, was intended, he said, to send "a clear and proportionate signal" to the Russian authorities.
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