The British ambassador to Moscow today denounced Russia over its continuing failure to extradite a former KGB officer for the killing of the Kremlin critic Alexander Litvinenko.
Sir Anthony Brenton said Anglo-Russia relations were not in crisis but there were sharp differences over the Litvinenko affair.
In an interview with the Interfax news agency and the Kommersant newspaper, Sir Anthony said: "It is surprising to me that the Russian authorities do not see their own national interest in putting on trial, in the place where most of the evidence and witnesses are available, a suspected murderer and carrier of highly toxic radioactive substances."
Britain has accused the ex-KGB agent Andrei Lugovoi of killing Mr Litvinenko last November in London with polonium 210, but Russia has said its constitution does not allow extradition of its citizens.
In a throwback to the cold war, Britain and Russia have engaged in tit-for-tat expulsions. Russia last week expelled four British diplomats following Britain's decision to send four Russian middle-ranking embassy officials home.
On the Lugovoi extradition request, Sir Anthony said Britain was not asking Russia to disobey its own constitution, "but to work with us creatively to find a way around this impediment, given the serious and unprecedented nature of this murder. Such cooperation has not been forthcoming."
Russia has offered to try Mr Lugovoi in Moscow if Britain presents sufficient evidence against him. Sir Anthony said this was not an option for British prosecutors who, he stressed, were independent of the government.
"They note that the crime was committed against a British citizen and took place in London. The appropriate venue for the trial is therefore London," he said.
Despite the tit-for-tat expulsions, Britain and Russia have sought to contain the crisis as both have considerable common economic interests.
Many Russian firms have listed in London and the UK energy giants Shell and BP have substantial investments in Russia.
But relations last week took another turn for the worse amid revelations of a second assassination conspiracy.
Senior Whitehall sources said a suspected assassin was intercepted in London last month before he could mount an attempt on the life of Boris Berezovsky, the Russian oligarch who has infuriated President Vladimir Putin with his open calls for insurrection in Moscow.
For its part, Russia today again claimed that Britain's expulsion of four Russian diplomats was politically motivated and unjustified.
Britain's reaction was "plainly groundless, inappropriate, unjustified and lies exclusively in a political framework", Alexander Zvyagintsev, Russia's deputy prosecutor-general, told a news briefing.
