Murder suspect a marked man.
Andrei Lugovoy, a former KGB agent, is charged with poisoning Litvinenko in London last year with radioactive polonium-210.
Alex Goldfarb said Lugovoy is a "marked man" who may mysteriously drop dead or disappear.
He said Lugovoy poses a bigger threat to the Kremlin than any diplomatic row with the UK over his extradition.
Lugovoy, who runs a bodyguard training business, said he is innocent of the poisoning and labelled the decision to charge him as politically motivated.
The Russian Foreign Ministry and the Prosecutor General's office said it is highly unlikely that Lugovoy would be extradited to Britain.
Speaking from New York, where he heads a civil liberties foundation set up by exiled tycoon Boris Berezovsky, Mr Goldfarb said Lugovoy is in danger.
He said: "More likely he will drop dead or become a fugitive and disappear or something. He might mysteriously drop dead.
"Why? Because the Government knows he holds in his hands the future of Mr Putin and the whole regime."