Prosecutors are demanding that a former KGB agent be extradited from Russia to stand trial for the murder of Alexander Litvinenko.
The move, which has the full support of Downing Street, is likely to ratchet-up already strained diplomatic tensions between Britain and Russia.
Director of UK Public Prosecutions, Sir Ken Macdonald said: "I have today concluded that the evidence sent to us by the police is sufficient to charge Andrey Lugovoy 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."
Describing the crime as "extraordinarily grave", Sir Ken said he had instructed lawyers to seek the early extradition of Andrey Lugovoy—a former KGB operative—from Russia to the United Kingdom.
Litvenenko died of radiation poisoning in London last November.
"He was found to have ingested a lethal dose of Polonium 210, a highly radioactive material," said Sir Ken.
"During his difficult, fatal illness and following his death, the Metropolitan Police Service in London conducted a careful investigation into how this had happened."
The process of that investigation—which took Scotland Yard officers into Russia—strained already prickly diplomatic relations between the UK and Russia.
The Russian ambassador was called into the Foreign Office on Tuesday morning, in the words of a Downing Street official, to "underline our view that Russia should comply with that legal request"
"Murder is murder," said the Prime Minister's spokesman, responding to questions about the diplomatic implications of the request by stressing the need for the international rule of law to be respected.
Russia has no formal extradition treaty specifically with the UK. The spokesman for the Prime Minister said the UK's extradition relations with Russia were based on the 2001 European Convention on Extradition and the 1957 Council of Europe convention
A spokeswoman for the Russian prosecutor's office said: "In accordance with Russian law, citizens of Russia cannot be turned over to foreign states."
She said that Lugovoi could be tried in Russia—with the evidence provided by the UK prosecution service.
The Prime Minister's Office did not want to comment on the issue, saying that rather than speculating, people should await the Russia's "considered legal response" to the formal request—due later in the week, after translation.
Lugovoi, formerly a platoon commander in the KGB, and now the head of a security firm, categorically denied the charges. "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," he was quoted as saying by RIA-Novosti and other agencies.
Many analysts describe diplomatic relations between the UK and Russia as being at the lowest ebb since the cold war.
The UK's refusal to extradite to Russia business tycoon Boris Berezovsky and the Chechen separatist envoy Akhmed Zakayev – both of whom were associates of Litvinenko – has added to the increased diplomatic tensions.
In a statement on his deathbed, Litvinenko himself directly accused Vladimir Putin, the Russian President, of being behind his murder: "You may succeed in silencing me, but that silence comes at a price. You have shown yourself to be as barbaric and ruthless as your most hostile critics have claimed."
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