Ken Macdonald, the head of Britain's Crown Prosecution Service, told a news conference in London that investigators have enough evidence to press criminal charges against Andrei Lugovoi, a wealthy Russian businessman with a past in the Soviet secret services.
"I have today concluded that the evidence sent to us by the police is sufficient to charge Andrei Lugovoi with the murder of Aleksandr Litvinenko by deliberate poisoning. I have further concluded that the prosecution of this case would clearly be in the public interest," Macdonald said.
The announcement ended months of suspense over the outcome of the British prosecutors' investigation into the high-profile killing, and threatens to further strain Russian-British relations.
Lugovoi, speaking to journalists in Moscow today, denied the allegations.
"I want to stress once again that, (a) I do not consider myself guilty; (b) on the contrary, I consider myself a victim, as my family members and I were subjected to a radiation attack on British territory and I have said it before and I'm still saying it with all firmness and responsibility; and (c) I consider the accusations brought against me today inadequate, and I don't even understand what kind of evidence they have or the motive why I supposedly did it and how I did it," he said.
Litvinenko died in a London hospital on November 23 after being poisoned with a rare radioactive substance, polonium-210.
On November 1, the day he fell ill, Litvinenko drank tea in the company of Lugovoi and another former KGB colleague, Dmitry Kovtun, at the Millennium Hotel in London. Tests revealed that the pot of tea was heavily contaminated with polonium, and several hotel staff have tested positive for low-level radiation contamination.
Kovtun, who lives in Russia, has also denied any involvement.
Macdonald said the Crown Prosecution Service will seek Lugovoi's extradition, saying: "I have instructed [Crown Prosecution Service] 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 here with murder and brought swiftly before a court in London to be prosecuted for this extraordinarily grave crime."
Foreign Secretary Margaret Beckett said Britain expects "full cooperation" from Moscow in bringing Lugovoi to face justice in London.
Lugovoi's prosecution, however, will be difficult to mount.
The 43-year-old Litvinenko was a fierce critic of Russian President Vladimir Putin, whom he blamed for his murder in a deathbed statement.
Moscow has dismissed the accusation as ridiculous. The Russian Prosecutor-General's Office has said that Russia will not extradite any Russian suspects to Britain in the case.
In addition, Russia and Britain do not have an extradition treaty and the Russian Constitution doesn't allow for the extradition of its citizens based in Russia.
A spokeswoman for the Russian Prosecutor-General's Office, Marina Gridneva, explained the legal situation to reporters today in Moscow.
"In accordance with Article 61 of the Russian Constitution, a citizen of the Russian Federation cannot be handed over to another state," she said. "A citizen who has committed a crime on the territory of a foreign country can be prosecuted on the basis of materials provided by that country, but only in Russia, if there is an analogous crime punishable under Russian legislation."
But Andrei Nekrasov, a Russian filmmaker and friend of Litvinenko, tells RFE/RL's Russian Service that having Lugovoi stand trial in Britain is not the main concern.
"This is a very positive development, we had long been waiting for it. It's important to tell the truth, regardless of the chances to bring the suspect to court," Nekrasov said. "Personally, think these chances are minimal. This is not an ordinary murder, it's a political murder about which one should speak openly. The truth about it will benefit both British and Russian society, regardless of the chances of extradition."
The Crown Prosecution Service's announcement deals a blow to diplomatic ties between London and Moscow, already strained by the politically charged case.
"Complications in Russian-British relations are inevitable," commented Yevgeny Volk, head of the Heritage Foundation think tank in Moscow. "A categorical refusal to hand over Lugovoi will be perceived negatively in Britain, it will be interpreted as an attempt to hide a murderer. Here in Russia, this case is regarded as political, and the fact that the British media has paid great attention to it is perceived as an attempt to tarnish Russia and its leaders."
Lugovoi said today that he plans to make further revelations that could intensify the scandal.
"We will probably express our position within a week," he said. "We will get prepared, invite journalists, possibly hold a news conference, and make statements about all those events that have taken place in recent years, in the past year, around Litvenenko and myself. And I think I can recall some facts that will cause a sensation for British public opinion."
Extradition has long been a sticking point in Russian-British relations.
Russia is angered by Britain's refusal to extradite London-based Russian tycoon and Kremlin critic Boris Berezovsky as well as Akhmed Zakayev, a top envoy of Chechnya's separatist leadership. Both were close to Litvinenko.
Berezovsky fanned tensions last month by calling for Putin's overthrow.
(with material from agency reports)