Robert Owen, chairman of the inquiry, said he would grant Dimitri Kovtun “core participation status” in the case if he meets a number of conditions including that he provides a full witness statement and discloses any relevant material.
Kovtun and Andrei Lugovoi are prime suspects in the murder of Litvinenko, 43, who died nearly three weeks after he consumed tea laced with polonium-210 on November 1, 2006, at the Millennium Hotel in London.
They deny involvement and remain in Russia, having initially refused to take part in the inquiry.
However, earlier this month Kovtun offered to give evidence via video link.
Robert said he would allow the suspect to give evidence to the inquiry on July 27, provided he submits all relevant material by May 22.
“I will not have these proceedings disrupted or manipulated as a consequence of the lateness of this application,” he added.
The inquiry has heard that Kovtun and Lugovoi would be arrested on suspicion of Litvinenko’s murder should either return to the UK.