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.
Russia's failure to extradite Andrei Lugovoi has angered Britain
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.
Their practical impact though may well be limited.
Certainly it will be harder for Russian officials to visit the UK; the position for ordinary Russian citizens will remain largely the same; though discussions to speed up existing visa procedures will be suspended.
Mr Miliband said the UK was sending a clear and proportionate signal
The man the British government wants to bring to trial - Andrei Lugovoi - will have his own travel plans restricted since he clearly will not be able to go anywhere there is an established extradition procedure with the UK.
Beyond that, and the inconvenience to the four diplomats expelled, that is as far as it goes.
But this has now become a highly charged political matter. At the very least an equivalent Russian response is widely expected.
What matters then will be whether the two governments can draw a line under this affair and begin to restore the tattered fabric of their relationship. Or will relations simply head into the freezer?
Given the improving level of economic ties between Britain and Russia, neither country has any fundamental interest in making a meal of this.
But given the gravity of what happened, the government in London clearly felt that it needed to make it clear that actions of this kind - whoever in Russia was responsible - are just not acceptable.
More worrying to many analysts is the way that this "local dispute" between London and Moscow plays into the wider pattern of relations between Russia and the West.
Mr Putin has launched a number of rhetorical attacks on the West
Russia's flexing of its energy muscle has already alarmed many Europeans who generally take a fairly negative view about domestic political developments in Russia.
Russian President Vladimir Putin does not take kindly to being lectured on matters like human rights and democracy and this sensitivity has added another element of tension into the mix.
Russia's broader desire to be treated as an equal by Washington on the world stage only complicates the picture further.
From Moscow's view, Washington and Nato are encroaching more and more into Russia's sphere of influence.
Russia wants Nato's expansion eastwards to go no further and it certainly does not want the US to deploy an anti-missile system in Poland and the Czech Republic.
Over the past several months, President Putin and other Russian spokesmen have launched a series of rhetorical attacks against the US and its allies reminiscent of the sort of insults bandied about during the Cold War.
This is the context within which the row between London and Moscow must be situated.
It would be wrong to speak of a new Cold War; the world has simply changed too much.
Ties with Russia had cooled before the UK's Tony Blair left office
But it is clear that the new Russia is far from comfortable with its present international position.
Russia and the West have so far failed to engage in a purposeful dialogue that satisfies the aspirations of both sides.
On present evidence it is hard to see the UK-Russia row as leading to a fresh start.
Both governments may wish to contain the damage, but the broader climate between Russia and the West is already chilly in the extreme.
So far though, all of the tensions have not influenced Russia's behaviour on the vital international issues of the day, not least efforts to contain Iran's nuclear ambitions.
Some diplomats fear that recent Russian statements on the political future of Kosovo could signal that the worsening atmosphere is now beginning to have a practical effect.
Will the UK-Russia row make things worse? Well, as one leading British diplomat noted, "it could mark a further toughening in the relationship".
"It is one thing [for Russia] to react bilaterally," he went on, "quite another to put security in the Balkans at stake."
So events in London could have diplomatic repercussions that extend way beyond the narrow sphere of UK-Russia ties.