By Gabriel Gatehouse
British police have questioned a Russian dissident who met the former Russian agent Alexander Litvinenko days before his murder in London.
Sidelnikov met Litvinenko two days before he was fatally poisoned
Andrei Sidelnikov told the BBC that Russia's security service tried to stop him from leaving Russia and that he is seeking asylum in the UK.
Mr Litvinenko died of poisoning from radioactive polonium-210 last year.
The main suspect of British police, Andrei Lugovoi, is in Russia and has immunity from prosecution as an MP.
Mr Sidelnikov arrived in London from the Ukrainian capital Kiev on Sunday.
He said he met Mr Litvinenko on 30 October 2006. Two days later, the former FSB agent was poisoned with a fatal dose of polonium.
Detectives from Scotland Yard have questioned him about the meeting.
The Metropolitan Police could not confirm whether they were treating him as a witness in the case.
"We met in a cafe near Oxford Circus," Mr Sidelnikov told the BBC. The pair discussed the murder of Anna Politkovskaya.
Ms Politkovskaya, a journalist who was critical of the Kremlin's role in Chechnya, had been shot dead in the entrance to her apartment in Moscow less than a month earlier.
Mr Litvinenko said 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.
Andrei Lugovoi is the main suspect of UK police in Litvinenko's death
The Russian authorities deny any involvement and no-one has been convicted in the case.
Mr Sidelnikov initially tried to fly out from Moscow, but was prevented from travelling by the FSB, the Federal Security Service.
He was given a signed document, seen by the BBC, which states that the FSB refused him permission to leave Russian territory, but gives no reason for the refusal.
He eventually made it to Ukraine overland, via neighbouring Belarus. From there he boarded a flight to London.
Andrei Sidelnikov is the leader of "Pora", an opposition youth movement which advocates an Orange-style revolution to overthrow the government of Vladimir Putin. He said he feared for his life if he remained in Moscow.
"I am very afraid about my life in Russia. When I saw the letter from the FSB, I thought that it was the end of my story in Russia."
The Russian embassy in London told the BBC they were aware of Mr Sidelnikov's arrival in the UK, but had no specific information about his case.
A spokesman said his decision to apply for political asylum was a matter for the British authorities. The Home Office said they do not discuss the details of individual applications.
Mr Sidelnikov's case comes at a time of strained relations between Russia and the UK.
Britain is seeking the extradition of Andrei Lugovoi, the man police suspect of the murder of Alexander Litvinenko.
Russia has refused to extradite Mr Lugovoi who, as a newly-elected member of the Russian parliament, has immunity from prosecution. He denies the charges.