The Russian prosecutor-general has ruled out extraditing former KGB officer Andrei Lugovoi to the UK over the murder of Alexander Litvinenko.
Andrei Lugovoi has denied the charges
"Extradition is out of the question, because it contradicts our constitution," Yuri Chayka was quoted by the Itar-Tass news agency as saying.
But Mr Chayka raised the possibility of a joint investigation by Britain and Russia into Mr Litvinenko's death.
The UK wants to charge Mr Lugovoi, who denies involvement, with murder.
He added that investigators in Russia were continuing to study material submitted by UK officials.
Mr Litvinenko died in November 2006 after exposure to the radioactive isotope polonium-210.
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 isotope was found in a string of places that Mr Lugovoi visited in London, but he said he was a witness, not a suspect in the case.
The UK's director of public prosecutions has recommended that Mr Lugovoi be tried for murder by "deliberate poisoning" and a formal extradition request has been submitted to the authorities in Moscow.
But Mr Chayka dismissed the prospect of Russia handing him over.
He said: "Britain has offered us no more than an analysis of its proofs, not the materials of the criminal case.
"If there's a necessity, we may send a request for additional information, but not for extradition, but for holding a person criminally responsible in the territory of the Russian Federation."
Mr Chayka added that he hoped to see "a possible joint investigation into certain episodes aimed at establishing the identity of the persons involved in this grave crime."