"We very strongly believe the Litvinenko case to have had some state involvement. There are very strong indications," the source told the BBC's Newsnight programme.

MI5 believes the FSB, Russia's security service, has its sights on several dissidents living in the UK. The officer told Newsnight that MI5 believed it had foiled a plot in June last year to kill the billionaire Boris Berezovsky, an outspoken critic of the former Russian president Vladimir Putin.

Mr Litvinenko died of radiation poisoning in a London hospital in November 2006. British detectives named the former KGB agent Andrei Lugovoi as the prime suspect but he denied any involvement and Russia has refused to extradite him to face trial.

Gordon Brown has raised the subject of Mr Lugovoi's extradition with the new Russian President, Dmitry Medvedev, at the G8 summit in Japan. However, an hour-long discussion failed to break the deadlock on issues including the closure of British Council offices and a dispute between BP and Russian oil companies.