The UK government has today condemned Russia's refusal to extradite murder suspect Andrei Lugovi.
Russian authorities formally confirmed that they would not cooperate with the extradition request.
The Crown Prosecution Service (CPS) wants Mr Lugovi to stand trial for the murder of the former KGB agent Alexander Litvinenko, who was poisoned with polonium-210 in London last November.
The Foreign Office said Russia's refusal was "unacceptable" and Downing Street said it was extremely disappointed at the decision, adding it is deeply regrettable Russia had not cooperated.
A spokeswoman said the Foreign Office would now consider its response "with the deliberation and seriousness that it deserves."
She continued: "We've consistently said the murder of Alexander Litvinenko was a serious criminal matter. Hundreds of British citizens and visitors to the capital were put at risk."
Russia said its constitution did not allow for extradition of citizens. Russian authorities did offer to try Mr Lugovi for the murder in his home country, but this option was dismissed by the CPS.
Sir Ken Macdonald, director of public prosecutions, said: "They have said that they are prepared to put Mr Lugovoi on trial in Russia if the evidence is forwarded to them.
"The allegation against Mr Lugovoi is that he murdered a British citizen by deliberate poisoning and that he committed this extraordinarily grave crime here in our capital city. The appropriate venue for his trial is therefore London."
Downing Street said the CPS had seriously considered Russia's offer but concluded it would be unacceptable, as witnesses and evidence are at the crime scene in the UK. Also, it could not be confident the trial would meet acceptable standards.
The CPS first made its extradition request in May and it was expected Russia would refuse to cooperate. Mr Lugovi continues to maintain his innocence.