The Kremlin has been accused of attempting to manipulate the Alexander Litvinenko inquiry after a prime suspect in the spy's killing failed to appear.
Dmitri Kovtun had been due to give evidence by video link from Moscow today but doubts over his position surfaced at the 11th hour.
It was confirmed that the Russian was not in a position to give evidence today and chairman Sir Robert Owen gave him a final deadline of 9am tomorrow.
Referring to the details of the latest developments outlined by Robin Tam QC, counsel to the inquiry, Sir Robert said: "The facts as you have outlined them give rise in my mind to the gravest suspicion that an attempt is being made to manipulate the situation so as to enable him hereafter to assert that he would have been willing to give evidence to assist me in this Inquiry, but has been unable to do so for reasons beyond his control."
Ben Emmerson QC, who represents the Litvinenko family, issued a damning assessment of the situation.
He said: "I can only endorse your concerns that it appears that these proceedings are being manipulated in a co-ordinated way between Mr Kovtun, the murderer, and the Russian state that sent him to commit the murder, a continuation of a collaboration that began in 2006.
"That said, we endorse your view that there should be no place left for Mr Kovtun or his masters in the Kremlin to hide behind procedural limitations as an excuse."
Kovtun originally indicated he would not engage with the inquiry when it opened earlier this year, before dramatically changing his mind.
Three days were set aside for his evidence but it emerged last week that he fears he may be committing an offence under Russian law if he gives evidence.
The inquiry was told that Kovtun believes he is bound by a non-disclosure agreement relating to an investigation in Russia.
Mr Tam said today that communications to the inquiry over the weekend indicate that he had not applied to authorities in his homeland to be discharged from his confidentiality obligations.
The inquiry heard Kovtun had suggested it was not for him to request that he is released from the non-disclosure clause, but rather that an application should be made by a third party.
However, in communications to the inquiry, Russia's ICRF (Investigative Committee of the Russian Federation) said that Kovtun was the only person who can apply to be discharged from the duty.
Kovtun had separately claimed the "obstacle" to him giving evidence is not a matter between himself and the ICRF, suggesting that instead arrangements needed to be made between Moscow and the UK, the hearing was told.
He was also quoted in media reports today as saying he had not obtained permission to give evidence and that he had been unable to contact the relevant investigator.
Kotvun dismissed suggestions that he was never serious about testifying, according to the BBC
Richard Horwell QC, for the Metropolitan Police, said: "It is very strange indeed that notwithstanding the purported signing of a non-disclosure agreement, and an obligation of confidentiality to the Russian investigators, Kovtun has no problem speaking to journalists."
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.
On his deathbed, Mr Litvinenko accused Russian president Vladimir Putin of ordering his assassination - which the Kremlin denies.