Businessman Dimitri Kovtun told a news conference in Moscow he thought the death was "suicide by negligence".
Mr Litvinenko, 43, died three weeks after drinking tea laced with the radioactive substance polonium-210.
English prosecutors suspect Mr Kovtun and Andrei Lugovoi of poisoning him at a London hotel in November 2006.
Before he died, Mr Litvinenko - a former officer with the Federal Security Service (FSB), the successor to the KGB - accused Russian president Vladimir Putin of ordering his assassination. The claims were denied by the Kremlin.
However, speaking at a news conference in Moscow, Mr Kovtun said he believed Mr Litvinenko may have poisoned himself.
"I am more than sure that he dealt with polonium without knowing it," the Russian news agency Interfax reported him saying.
"Maybe it was leaking and polonium accumulated in his body gradually. It is possible that something he carried with him led to a gradual accumulation of polonium in the body," he added.
Mr Kovtun and former KGB bodyguard Mr Lugovoi have denied any involvement and have remained in Russia, despite an ongoing public inquiry into Mr Litvinenko's death.
Sir Ken Macdonald, who was then director of public prosecutions, had recommended in May 2007 that Mr Lugovoi should be charged with the murder of Mr Litvinenko.
Last month, Mr Kovtun offered to give evidence to the inquiry, at the Royal Courts of Justice in London, via video link.
Inquiry chairman Sir Robert Owen said he would grant Mr Kovtun "core participant" status to give evidence, if he met a number of conditions - including the provision of a witness statement and any relevant evidence.
Russian spy Alexander Litvinenko may have accidentally killed himself after handling radioactive material, a key suspect in his death has said.