A Russian former KGB officer should be charged with the murder by poisoning of Alexander Litvinenko, the UK's director of public prosecutions has recommended.
Sir Ken Macdonald said Andrei Lugovoi should be tried for the "grave crime".
Mr Litvinenko, 43, an ex-FSB agent and a critic of Russian President Vladimir Putin, died in London last November.
Mr Lugovoi denied any involvement and said the charges against him were "politically motivated"; the Kremlin said he would not be extradited.
Mr Lugovoi met Mr Litvinenko on the day he was taken ill.
Radioactive isotope polonium-210 - the substance found in Mr Litvinenko's body - has been detected in a string of places Mr Lugovoi had visited in London.
But Mr Lugovoi has insisted he was a witness and a victim but not a suspect.
"I consider that this decision to be political, I did not kill Litvinenko, I have no relation to his death and I can only express well-founded distrust for the so-called basis of proof collected by British judicial officials," Russian news agencies quoted Mr Lugovoi as saying.
The formal submission of a request for Mr Lugovoi's extradition is expected to take place before the end of the week, after it has been translated.
A spokesman for the Kremlin said Russia's constitution did not allow its nationals to be extradited.
The spokesman added it was waiting for the "British side to actually do something rather than make statements".
The Russian general prosecution service also said there was "no way" Mr Lugovoi could be extradited because of constitutional constraints.
But the service's spokesman added that a Russian citizen who had committed a crime in another country "should be prosecuted in Russia with evidence provided by the foreign state".
UK Foreign Secretary Margaret Beckett said she had told the Russian ambassador that she expected "full co-operation" with regards extraditing Mr Lugovoi.
And Prime Minister Tony Blair's official spokesman said people should wait and see what Russia's "considered legal response" was to the extradition request.
He pointed out that in 2001 Russia had signed the 1957 EU convention on extradition.
Mr Litvinenko, who was granted political asylum in the UK in 2000 after leaving Russia and went on to take British citizenship, died at University College Hospital on 23 November.
Sir Ken Macdonald told a news conference: "I have today concluded that the evidence sent to us by the police is sufficient to charge Andrei Lugovoi with the murder of Mr Litvinenko by deliberate poisoning.
"I have further concluded that a prosecution of this case would clearly be in the public interest.
"In those circumstances, I have instructed CPS lawyers to take immediate steps to seek the early extradition of Andrei Lugovoi from Russia to the United Kingdom, so that he may be charged with murder - and be brought swiftly before a court in London to be prosecuted for this extraordinarily grave crime."
Mr Litvinenko's widow Marina said that she welcomed the decision on what was a "big day" for her.
She said: "I am now very anxious to see that justice is really done and that Mr Lugovoi is extradited and brought to trial in a UK court."
She added that any court case should be held in Britain, and that she believed more than one person was responsible for her husband's death.
The counter-terrorism command of the Metropolitan Police has been conducting a detailed international investigation into Mr Litvinenko's death. The police inquiry, during which officers followed a trail of polonium radioactivity at a series of locations visited by Mr Litvinenko in London before he died, eventually took them to Moscow.
His friends, including London-based Russian tycoon Boris Berezovsky, have accused the Kremlin of ordering his assassination but the Russian government has rejected such claims.
Police passed a file to the Crown Prosecution Service in January.