Litvinenko died of radioactive poisoning in London on November 23, 2006, three weeks after suddenly falling ill. British investigators accused Russian agent-turned-businessman Andrei Lugovoi over the murder, and demanded his extradition, sparking a major diplomatic row with Moscow.
"Britain's position remains clear - Lugovoi must face trial in the U.K.," Anne Pringle told a news conference in Moscow.
Lugovoi, a former KGB operative who owns a multi-million dollar private security firm, met with Litvinenko in London before the ex-FSB officer fell ill, but denies any involvement and says Litvinenko tried to recruit him for the British Secret Intelligence Service (MI6).
Lugovoi was elected to Russia's parliament in December last year, as a member of the ultra-nationalist Liberal Democratic Party. His status as a lawmaker gives him immunity from prosecution in Russia.
Large traces of radioactive polonium-210 was found in Litvinenko's body, but British authorities have not yet made public any official document specifying the exact cause of his death or the results of the autopsy.
Russia has long been demanding from the U.K. additional evidence incriminating Lugovoi, but the newly-appointed ambassador said on Thursday that "London has already submitted sufficient evidence to extradite him to Britain."
Russia's top investigators said in July they have made substantial progress in investigating the killing of Litvinenko, and rejected claims of the Kremlin's role in his death.
Litvinenko was fired from the FSB (formerly the KGB) following a 1998 press conference in which he and a number of other FSB officers alleged that they had been ordered to murder and kidnap a number of high-profile figures.
He received British citizenship in 2006 and published two books in the U.K. alleging the involvement of the Russian security services in a series of apartment bombings in Russia in 1999.
