Before his death, Litvinenko accused Russian President Vladimir Putin of ordering his killing.
One of two Russians accused of murdering ex-KGB spy Alexander Litvinenko with a radioactive poison in London in 2006 has been given less than 24 hours to agree to appear via videolink before a British public inquiry into the death.
British authorities say there is evidence to prove Dmitry Kovtun, along with fellow Russian Andrei Lugovoy, poisoned Kremlin critic Litvinenko with green tea laced with polonium-210 at the Millennium Hotel in central London.
Before his death, Litvinenko accused Russian President Vladimir Putin of ordering his killing. However, the Kremlin has always denied any involvement, as have Lugovoy and Kovtun, whom Russia has refused to extradite.
In March, Kovtun, who initially refused to take part in the British inquiry applied for "core participant" status, given to someone who might have played a key role in the events being investigated. He was due to start giving evidence by videolink on Monday.
However, last week he contacted the inquiry to say he had received legal advice that he was bound by an obligation of confidentiality to an ongoing Russian investigation into the death, and that if he appeared without the permission of the Russian authorities he would be committing an offence.
The inquiry chairman Robert Owen said on Monday the late intervention "gave rise to the gravest suspicion" that Kovtun was attempting to manipulate the situation to give the appearance he was willing to cooperate.
He gave Kovtun a deadline of Tuesday at 9 a.m. (0800 GMT) to agree to give evidence.
"I have decided, albeit reluctantly, that a final opportunity must be given to this man that he wants to live up to his assertion that he wants to assist me in this inquiry," Owen said.