A suspect in the killing of Alexander Litvinenko will not give evidence at the inquiry into the spy's death, according to reports.
Dmitri Kovtun was due to start giving his evidence by video-linkfrom Moscow today.
But doubts emerged on Friday over his participation and now he has reportedly confirmed that he will not now take part, saying he had been unable to get permission from the Russian authorities.
Speaking to the BBC in Moscow, Kovtun said he had been unable to contact the relevant Russian investigator.
"Even if the investigator, who I have not been able to reach, gave me permission, he would still have to make a decision on how much information I could disclose," the Russian said.
He claimed he had only received a response to a question about the legality of giving evidence from London on Friday and dismissed suggestions that he was never serious about testifying, according to the BBC.
Last week the inquiry was told that Kovtun believes he may be committing an offence under Russian law if he gives evidence.
Robin Tam QC, counsel to the inquiry, said his position appeared to be that he regarded himself as bound by obligations of confidentiality to a Russian investigation.
"He has suggested that, unless he is discharged from that obligation by the Russian investigating body, he regards himself as unable to give evidence to the inquiry next week."
Richard Horwell QC, for the Metropolitan Police, said: "None of this comes as any surprise.
"It appears Kovtun's request to give evidence was nothing more than an attempt to become a core participant and obtain as much information about these proceedings as he could."
Sir Robert gave Kovtun a deadline of noon today to say whether he would take part.
Mr Litvinenko, 43, died nearly three weeks after drinking tea laced with polonium-210 in London in November 2006.
Police concluded that the fatal dose was probably consumed during a meeting with Kovtun and Andrei Lugovoi at a hotel in central London.
British authorities later decided that Lugovoi and Kovtun should be prosecuted for murder.
However, attempts to extradite the pair - who deny involvement - have failed and they remain in Russia.
Both originally refused to take part in the probe, which is sitting at the Royal Courts of Justice in London.
However, in March Kovtun dramatically changed his mind and offered to give evidence.
Sir Robert said he would grant him core participation status and allow him to give evidence if he met a number of conditions.
On his deathbed, Mr Litvinenko accused Russian President Vladimir Putin of ordering his assassination - which the Kremlin denies.
In April Kovtun said he believes the former KGB officer might have killed himself accidentally after handling radioactive material.