Russia's Prosecutor General Office said Andrei Lugovoy, also a former KGB man, would not be extradited over the alleged "deliberate poisoning" of Mr Litvinenko in a London hotel six months ago because any charge against him could be dealt with on home soil.

But Tony Blair insisted that the UK would "not in any way shy away" from trying to ensure justice prevails and Foreign Secretary Margaret Beckett said she expected "full co-operation" from Russian authorities.

Director of Public Prosecutions Sir Ken Macdonald QC said the murder was an "extraordinarily grave crime" and a prosecution would "clearly" be in the public interest.

Sir Ken said he had instructed CPS lawyers to take immediate steps to seek Lugovoy's extradition so he could be "brought swiftly before a court in London".

Attorney General Lord Goldsmith QC said he had been consulted and backed the CPS decision.

Mr Litvinenko, 43, died from a fatal dose of the radioactive agent Polonium 210 last November.

On his death bed, he accused Russian President Vladimir Putin of being responsible for his death, saying: "May God forgive you for what you have done, not only to me, but to beloved Russia and its people."

Lugovoy, who met Mr Litvinenko in the Pine Bar of the Millennium Hotel in Grosvenor Square on November 1, the day he was allegedly poisoned, is still protesting his innocence, saying detectives have "no evidence" and should search for their culprit in London rather than Moscow.

Mr Litvinenko's widow Marina welcomed the British move and vowed not to rest until her husband's killer was brought to justice in the UK.