A suspect in the killing of Alexander Litvinenko must finally confirm whether or not he will give evidence at the inquiry into the spy's death.
Dmitri Kovtun was due to start giving testimony from Moscow by video link today after he was granted "core participant status" by chairman Sir Robert Owen.
Three days were set aside for his evidence but on Friday doubts emerged over whether the Russian would give evidence.
Kovtun believes he may be committing an offence under Russian law if he does so, the probe was told.
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."
Today's hearing will begin three hours later than planned after Sir Robert gave Kovtun a noon deadline to say whether he will take part.
Litvinenko, 43, died nearly three weeks after consuming 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 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 that he would grant him core participation status and allow him to give evidence if he meets a number of conditions.
Last month it was confirmed that Kovtun would give evidence after he supplied details to the probe.
He disclosed that he intended to assert privilege against self incrimination when giving testimony.
This means that he may decline to answer any question that he believes could incriminate him.
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.