Three pathologists are conducting the postmortem - one from the British Home Office, another working on behalf of the Litvinenko family, and a third as an independent expert.
It is not yet known when the final results of the examination will be made public.
Litvinenko, a Russian intelligence defector, severe critic of President Vladimir Putin's administration and a close associate of exiled oligarch Boris Berezovsky, died in a London hospital with symptoms of radioactive poisoning. British health officials said a large dose of polonium-210, a toxic uranium by-product, was found in his body.
British Airways announced Wednesday that radiation contamination has been detected on two of their planes that traveled the Moscow-London route, and that they have been grounded at London's Heathrow Airport.
A third BA plane was grounded at Moscow's Domodedovo Airport on suspicion of contamination. On Friday, it was cleared to fly to London without passengers, where it will undergo similar tests.
Following Litvinenko's death, the Western press circulated a message purporting to be his deathbed note, in which he accused President Vladimir Putin of orchestrating his death, an allegation the president dismissed.
