The Cold War had been followed by a tepid peace, but the relationship between London and Moscow took a distinctly chilly turn yesterday with the formal request from the director of the Crown Prosecution Service for the extradition of a former KGB bodyguard over the murder of Alexander Litvinenko.

Because of the politically murky background to this case and the cast of characters involved, it is important to keep in mind one central fact: a man who had been granted full political asylum by the British government was deliberately killed in London by a lethal dose of radiation. The pallid former Federal Security Service officer turning his penetrating gaze at the camera as he faded away in a hospital bed was one of the enduring images of last year. There were fears that potential damage to bilateral relations could inhibit the British investigation into this dreadful crime. The decision last December by the Serious Fraud Office to halt an inquiry into the relationship between Britain's biggest arms manufacturer, Bae Systems, and the Saudi government after "consultations" between the Prime Minister and the attorney-general, showed what can happen when political relationships are allowed to take precedence over the rule of law. As recently as last week-end there were suggestions that the British Foreign Secretary was putting pressure on the prosecution service not to bring charges in the Litvinenko case. Fortunately, these fears have proved to be unfounded.

Russo-British relations are now in for a rough ride. Already there are tensions over a variety of issues, ranging from Russian oligarchs launching attacks on the Putin regime from British soil to disagreements over sanctions against Iran. There is also nervousness in the west about being seen to bait the Russian bear that controls so much of the world's future oil and gas supplies.

Vladimir Putin has been able to exploit this nervousness and his role as the ally of George Bush in the "war on terror", using this handy template to excuse gross human rights abuses in the war in Chechnya, erode basic freedoms and subvert the rule of law. Despite certain outward trappings, Russia has looked less and less like the liberal democracy it purports to be but has continued to shrug off criticism. In this respect, the gunning down of journalist and human rights campaigner Anna Politkovskaya may have proved a seminal moment. Mr Litvinenko was investigating her murder when he first fell ill.

Never for one moment must Mr Putin be allowed to believe he or his agents can act abroad with the same impunity he appears to enjoy at home. That is why the British government needs to stick by what should be seen as a classic case of the rule of law being applied in a criminal case.

Britain is in for a long, cold wait. There was some argument yesterday as to whether an extradition treaty applied between the two countries. The Kremlin's stance was that under the Russian constitution, its nationals cannot be handed to another state. Nevertheless, Britain's move forces Mr Putin on to the defensive and sends the message that if Russia wants to have dealings with the west, it must play by its rules. Political asylum is worthless if Britain cannot protect refugees against those who would silence them.