LONDON - Alexander Litvinenko was fooled by his alleged killer because he was "naive like a child", his father said on Thursday as the Russian's family prepared to mark the anniversary of his death from polonium poisoning.

Friends and relatives will visit the grave of Litvinenko in London's Highgate cemetery on Friday, a year to the day after the Kremlin critic-in-exile was killed by radioactive poison slipped to him in a cup of tea.

Father Walter Litvinenko said his son had trusted Andrei Lugovoy, the man accused by British prosecutors of murdering him, because both were former officers of the Soviet KGB who had fallen foul of the Moscow authorities and served jail terms.

Walter Litvinenko renewed accusations that responsibility lay ultimately with President Vladimir Putin and "the Lubyanka", a reference to the Moscow headquarters of the FSB security service, heir to the KGB.

Alexander Litvinenko, known as Sasha, "was a very experienced investigator but he was sometimes naive like a child", Walter told a news conference.

"When he saw this Lugovoy, who had also been a prisoner of the Lubyanka, of course he didn't realise this was someone sent by the FSB and he tried to help him, which was the main reason for Sasha's death."

Litvinenko fell violently ill after a business meeting with Lugovoy on November 1 at a London hotel last year, and died three weeks later in hospital.

In a case which has caused deep damage to relations, Britain has sought the extradition of Lugovoy but Moscow has refused, saying this is impossible under its constitution. It has strongly denied allegations of Kremlin involvement in the death.

In an interview with Reuters on Thursday, Lugovoy again denied killing Litvinenko. "British intelligence services are behind this," he said.

He is standing in Russia's December 2 parliamentary election as a candidate for the nationalist Liberal Democratic party, which Walter Litvinenko called part of a "three-headed monster", along with pro-Kremlin United Russia and Fair Russia.

"They're trying to seize ultimate power, grab Russia by the throat, using the naivety of the Russian people," he said.

Litvinenko senior rained insults on Putin, accused President George W. Bush of "falling in love" with the Kremlin leader and saying that appeasing Putin would "end very badly" for the West.

He was appearing at a book launch for "Allegations", a collection of Alexander Litvinenko's writings and interviews. It includes an August 2006 conversation with Radio Liberty in which he spoke of the possibility that Moscow would kill him in order to silence his criticism.

"If they listen to me now, let them know: I hire no bodyguards to protect myself, and I never hide anywhere. I live very openly, all the journalists know where to find me," Litvinenko said. "So, gentlemen, if you come to Britain to kill me, you will have to do that openly."