LONDON — A major diplomatic row loomed last night after Prime Minister Blair demanded the extradition from Russia of the man accused of murdering a former security service agent, Alexander Litvinenko.

Andrei Lugovoi was formally accused by British authorities yesterday of deliberately poisoning Litvinenko, a critic of President Putin of Russia, with the radioactive agent polonium 210.

The director of public prosecutions, Sir Ken Macdonald, backed by senior politicians, said there was enough evidence to charge Mr. Lugovoi over what he called an “extraordinarily grave crime.” He said steps would be taken to secure his extradition.

The announcement, almost exactly six months after Litvinenko died from radiation poisoning in a London hospital, marked a decision by the government not to back down in the face of enormous economic and diplomatic pressure.

Mr. Blair’s spokesman said: “Murder is murder — this is a very serious case. Nobody should be under any doubt as to the seriousness with which we are taking this case. Obviously we have political and economic connections with Russia, and Russia clearly plays an important role in international affairs. However, what that doesn’t in any way obviate is 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. That is the basis on which we proceed.”

Prosecutors stopped short of any accusations against the Russian state, but officials in Moscow quickly made it clear that Mr. Lugovoi would not be handed over to face British justice.

That leaves the possibility of a show trial in which Mr. Lugovoi may be used to shift the blame to enemies of the Kremlin.

After yesterday’s announcement, Mr. Lugovoi claimed the attempted prosecution was politically motivated and promised to disclose “sensational details about the case.”

“I did not kill Litvinenko,” he told Russian journalists. “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 promised a statement that would be “sensational for public opinion in Britain and that would fundamentally change the perceptions of the events that have occurred in the past years in Britain surrounding certain individuals of Russian origin.”

Russian prosecutors are running their own investigation into Litvinenko’s death and visited Britain in March to interview the exiled billionaire Boris Berezovsky, who also has been a fierce critic of Mr. Putin and who employed Litvinenko.

They have been frustrated in their attempts to have Mr. Berezovsky extradited to Moscow on a range of charges since he was granted political asylum in 2003.

Litvinenko was also granted asylum after claiming that he had been told by his bosses in the FSB — the successor to the KGB — to assassinate Mr. Berezovsky.

Litvinenko’s widow, Marina, made it clear that she would not welcome a trial in Russia. She said an extradition would be the clearest way for Mr. Putin to prove there was no state involvement in the death of her husband.

Mrs. Litvinenko had a 40-minute meeting with the Russian ambassador, Yuri Fedotov, and told him Russia’s cooperation “would prove there wasn’t state involvement.”

Earlier, Mr. Fedotov had been called in to see the British foreign secretary, Margaret Beckett. Afterward, Mrs. Beckett said: “We expect full cooperation from the Russian authorities in bringing the perpetrator to face British justice.”

The decision to seek the extradition of Mr. Lugovoi, a former member of the KGB, has been taken at the highest level and in full awareness of the row it would create.