British prosecutors accused a former KGB agent yesterday of murdering dissident Alexander Litvinenko with radioactive polonium and sought his extradition, throwing London and Moscow on to a diplomatic collision course.

The Crown Prosecution Service said it wanted to bring suspect Andrei Lugovoy before a British court and charge him with the "extraordinarily grave crime" of murdering exiled Russian Litvinenko in London last November.

Britain immediately summoned the Russian ambassador and told him in strong terms it expected "full cooperation," but a Moscow judicial source ruled out handing over Lugovoy.

"No one should be under any doubt about the seriousness with which we regard this case. Murder is murder," Prime Minister Tony Blair's spokesman said.

He stressed Britain had important political and economic ties with Russia but added: "This doesn't in any way obviate the need for the international rule of law to be respected, and we will not in any way shy away from trying to ensure that happens in a case such as this."

The murkiest case of murder and espionage since the Cold War has already strained diplomatic relations, and the extradition move looked certain to aggravate tensions further. Ties between Russia and the European Union are also frosty.

A source at the Russian prosecutor-general's office told RIA Novosti news agency: "Under the constitution of the Russian Federation, Russian citizens cannot be handed over to foreign countries for prosecution and Lugovoy appears to be a Russian citizen."

Litvinenko, a former KGB agent who had become a fierce Kremlin critic in exile, met Lugovoy and another Russian businessman, Dmitry Kovtun, at the Pine Bar of London's Millennium Hotel on November 1 last year.

Within hours, he had fallen severely ill. He suffered an agonizing death over the next three weeks as his organs gradually failed, his hair fell out and images of his emaciated body, hooked up to medical tubes, were published around the world. Doctors eventually diagnosed polonium poisoning.

In a letter dictated on his deathbed, Litvinenko, who had acquired British citizenship weeks before he was poisoned, accused Russian President Vladimir Putin of his murder.

Moscow dismissed the accusation as ridiculous. It has launched its own investigation into Litvinenko's death and denies that its security services played any part.

Lugovoy, a former KGB bodyguard who later worked as head of security for tycoon Boris Berezovsky, has previously laughed off reports Britain would seek his extradition. He has denied killing Litvinenko and accused British media of demonizing him.

Lugovoy was not answering calls yesterday and did not respond to messages sent to his cellular telephone.

A CPS spokeswoman said it would ask the police to obtain an arrest warrant, which would then be sent to Russian authorities.

Editor: Yan