"It's perfectly possible that his death was just an unfortunate accident - and Britain's MI6 intelligence service found itself in a ridiculous situation," Dmitry Kovtun, a former state security agent who met with Litvinenko several times before he fell ill, told a RIA Novosti news conference.
Andrei Lugovoi, a former Kremlin bodyguard-turned businessman who Scotland Yard is treating as the main suspect in the case, earlier said Litvinenko had been on the MI6 payroll and could have been killed either by the service itself or with its connivance.
Also speaking at the news conference, Lugovoi said: "This is a serious possibility that needs checking. What if Litvinenko grew careless with polonium? How was his former boss [in MI6] supposed to explain his death?"
London has sought Lugovoi's extradition saying Litvinenko may have received a fatal dose of polonium-210 when he met with Lugovoi and Kovtun at a luxury hotel on November 1 last year. Moscow has refused his extradition citing the Constitution, but has said Lugovoi could be tried at home if Britain provides sufficient evidence.
The extradition issue sparked a tit-for-tat row involving expulsions of British and Russian diplomats and visa restrictions earlier this year.
At the news conference Lugovoi repeated his earlier theories on the case, saying the Kremlin's enemies, including fugitive tycoon Boris Berezovsky and Chechen separatist emissary Akhmed Zakayev, both living as political emigres in London, could have plotted Litvinenko's death to discredit the Russian authorities.
Lugovoi, a millionaire with his own private security firm, plans to run for a seat in the lower house of parliament in the December elections, which would give him immunity from prosecution. He was upbeat about his chances of being cleared of the charges.
"I believe the truth will prevail. We will prove that we have nothing to do with the case. Britain will have to acknowledge that sooner or later," he said.
