Dissident quizzed over spy's murder

Mr Litvinenko died in London's University College Hospital on November 23 last year, three weeks after drinking tea laced with radioactive polonium-210.

Andrei Sidelnikov told the BBC he met Mr Litvinenko on October 30 2006, two days before he was poisoned. "We met in a cafe near Oxford Circus," Mr Sidelnikov said.

He said they discussed the murder of Anna Politkovskaya, a journalist who was critical of the Kremlin's role in Chechnya, who had been shot dead in the entrance to her apartment in Moscow less than a month earlier.

He said Mr Litvinenko told him he was expecting some documents from Moscow that might prove his suspicion that the FSB - the main successor to the KGB - was involved in her killing.

Mr Sidelnikov arrived in London from the Ukrainian capital Kiev on Sunday. He said Russia's security service had tried to stop him from leaving Russia and added that he is seeking asylum in the UK.

He said detectives from Scotland Yard have questioned him about the meeting with Mr Litvinenko.

A spokeswoman for Scotland Yard would neither confirm nor deny that Mr Sidelnikov had been questioned. "We are not prepared to discuss who we would want to interview with any investigation," she said.

The main suspect, Andrei Lugovoi, is in Russia. He denies the charges and, as a member of the Russian parliament, has immunity from prosecution. Russia has refused to extradite Mr Lugovoi.

The Russian authorities deny any involvement in Mr Litvinenko's death and no one has been convicted in the case.