Claims that the Kremlin backed the murder of ex-KGB agent Alexander Litvinenko are "preposterous", the Russian ambassador to the UK has said.
Yury Fedotov said the Russian courts could pursue the case
Yury Fedotov insisted Russia had no role in the "heinous" poisoning.
He added that Russia's constitutional ban on extraditing its own citizens could not be overturned on an "arbitrary and exceptional basis".
Four Russian embassy staff have been expelled over Moscow's refusal to hand over the chief suspect for the murder.
Former KGB agent Mr Litvinenko died of exposure to radioactive polonium-210 in London in November 2006.
Prosecutors want Andrei Lugovoi, an ex-KGB officer who denies involvement, to face trial in the UK.
KEY EVENTS IN CASE
1 November 2006: Alexander Litvinenko meets Andrei Lugovoi and another Russian at a London hotel
23 November 2006: Litvinenko dies in a London hospital
24 November 2006: A Litvinenko statement accuses Russian President Vladimir Putin of involvement in his death. Experts say Litvinenko was poisoned
6 December 2006: UK police say they are treating the death as murder
22 May 2007: Lugovoi should be charged with Litvinenko's murder, British prosecutors say
28 May 2007: UK makes formal request for Lugovoi's extradition from Russia
The radioactive isotope used to poison him was found in several places that Mr Lugovoi had visited in London.
In a letter to the Times newspaper, Mr Fedotov said Russian President Vladimir Putin had condemned the killing and offered to pursue the case through Russia's courts.
He wrote: "The Russian government values its relations with the UK and respects its laws and constitutional arrangements.
"A close and constructive relationship, of course, requires that the British government does the same."
Under the Council of Europe European Convention on Extradition 1957, the Russians have the right to refuse the extradition of a citizen.
The Convention gives the UK the right to request that the investigation be taken on by the Russian authorities.
However, Sir Ken Macdonald, director of public prosecutions, rejected an offer from Moscow to try Mr Lugovoi in Russia.