The Russian man the UK wants to charge with murdering Alexander Litvinenko has denied any involvement and said the case is "politically motivated".
Andrei Lugovoi, a former KGB officer, told Russian TV: "I'm a victim not a perpetrator of a radiation attack."
The UK's director of public prosecutions has recommended Mr Lugovoi be extradited for the "grave crime" of murder by poisoning of Mr Litvinenko.
He died in London last November after exposure to a radioactive isotope.
Mr Litvinenko, 43, was a former FSB official and had been a critic of Russian President Vladimir Putin since defecting to the UK.
The Kremlin earlier said Russia's constitution did not allow for Mr Lugovoi to be extradited.
But Prime Minister Tony Blair's official spokesman said people should wait and see what Russia's "considered legal response" was to a formal extradition request.
Mr Lugovoi told Russian television: "I've said it before and I'll say it again - my family and myself were attacked when we were in the UK.
"I think today's charges are completely inadequate. I don't understand what proof they have or what motive they think I might have - or, indeed, how I could have done it.
"So I'm deeply surprised about the inadequate actions of the British law enforcement bodies."
Mr Lugovoi met Mr Litvinenko on the day he fell ill.
Radioactive polonium-210 - the substance found in Mr Litvinenko's body - was later found in a string of places Mr Lugovoi had visited in London, but he has insisted he was a witness and a victim but not a suspect.
Mr Lugovoi said: "Within the next week we'll make a statement regarding events in which Litvinenko and myself were involved last year. I think I'll say a few things which will be sensational to a British audience. "
The formal submission of a request for Mr Lugovoi's extradition is expected to take place before the end of the week.
Echoing the Kremlin's comments, 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 to extraditing Mr Lugovoi.
And Mr Blair's official spokesman 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 London's University College Hospital on 23 November.
Director of public prosecutions Sir Ken Macdonald told a news conference he had instructed CPS lawyers to take immediate steps to seek the early extradition of Mr Lugovoi so "he may be charged with murder and be brought swiftly before a court in London to be prosecuted for this extraordinarily grave crime".
"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."
The counter-terrorism command of the Metropolitan Police has been conducting a detailed international investigation into Mr Litvinenko's death. Police passed a file to the Crown Prosecution Service in January.