Britain's ambassador in Moscow has officially requested the extradition of a chief suspect in the poisoning of a former Russian intelligence agent.

Ambassador Anthony Brenton submitted the documentation seeking the extradition of Russian businessman and former intelligence agent Andrei Lugovoi to Russia's Foreign Ministry.

British prosecutors said last week that they want to bring Lugovoi to trial in Britain for the murder of Alexander Litvinenko, who died late last year after being poisoned with polonium-210. Lugovoi says he is innocent.

Russian authorities say their country's constitution prohibits extraditing citizens.

The British request followed a six-month probe into Litvinenko's death.

British police determined that Litvinenko met with Lugovoi and another Russian businessman at a London hotel November first -- the day Litvinenko fell ill.

In a letter on his deathbed, Litvinenko accused Russian President Vladimir Putin of having ordered his poisoning. The Kremlin denies any involvement.

Lugovoi last week called the British charges politically motivated.

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