Scientists in London are performing an autopsy on former Russian spy Alexander Litvinenko, who died in London last week from radiation poisoning.

Three pathologists are attending Firday's autopsy in a London hospital. One represents Litvinenko's family, another is attending for the Russian government and the third is an independent specialist in case of a criminal prosecution.

British authorities say the examiners are wearing full protective clothing to avoid contamination by the highly radioactive substance polonium 210 found in Litvinenko's body. He died eight days ago after saying he was poisoned for his opposition to Russian President Vladimir Putin. The Kremlin denies the allegations.

Meanwhile, Russian Foreign Minister Sergei Lavrov told reporters Friday he is waiting for "concrete questions" from British authorities investigating Litvinenko's death. He says Russia is prepared to answer all questions thoroughly.

Earlier Friday, British Airways said one of its planes has been cleared to fly to London from Moscow for a radiation check.

Officials found traces of the toxic element believed to be the cause of Litvinenko's death in two other British Airways planes in London.

Also Friday, friends of Mr. Litvinenko released a letter from a former Russian security officer, Mikhail Trepashkin, saying that he had warned Litvinenko of a possible death threat years ago. The former officer, now imprisoned in Russia on spy charges, says Russian security forces had plotted to kill Litvinenko and other Kremlin foes.

Some information for this report was provided by AFP, AP and Reuters.