Sensitive evidence alleged to expose poisoned spy Alexander Litvinenko's ties to MI6 will be examined in secret, a coroner has ruled.
Lawyers for the former KGB agent's family believe the files may contain the key to his assassination in London in November 2006.
On Wednesday, his widow Marina spoke of her disappointment that such material was still being shrouded in secrecy amid claims the government planned to "suppress" evidence to protect relations with Russia.
Mr Litvinenko, 43, was poisoned with polonium-210 while drinking tea at the Millennium Hotel in London's Grosvenor Square.
Marina Litvinenko believes evidence is being hidden
His family believes he was working for MI6 at the time and was killed on the orders of the Kremlin.
They have urged the government to reveal documents which they believe would support this theory.
But foreign secretary William Hague has argued the disclosure of certain files relating to the case could pose a risk to national security.
Coroner Sir Robert Owen today told a pre-inquest review, at London's Royal Courts of Justice, he would consider a selection of that evidence in private, giving Mr Hague's application the "most stringent and critical examination".
He assured interested parties he would carry out a "full, fearless and open investigation into the circumstances of Mr Litvinenko's death".
Speaking after his ruling, Mrs Litvinenko said: "It isn't an ideal decision for us but it could have been worse".
She added: "I have to trust the coroner, I must trust him, I have no option. I believe he will do exactly what he says."
The nature of the evidence contained with files remains unclear but lawyers for the Litvinenko family claim the documents could point towards Russian state involvement.
Alex Goldfarb, a friend of the Litvinenko family, said: "Of course we are disappointed, not that much in his (the coroner's) decision but with the intention of the state to suppress the evidence which we believe is wrong and dictated by reasons which have nothing to with justice but have to do with trade and politics and international relations which is wrong.
"After all, it isn't regular economic crime or corruption, it's murder and a terrorist act and we will not stop until those killers, who we believe are sitting in the Kremlin, and those who are trying to appease them are exposed."
The inquest was due to formally open on 1May, more than six years after Mr Litvinenko, 43, was killed.
Former KGB agents Andrei Lugovoy and Dmitry Kovtun have been accused of his murder.
Prosecutors named Lugovoy as the main suspect in the case but Russia has refused to extradite him to the UK for questioning. Both men deny involvement.
A previous hearing was told Mr Litvinenko had been hired by MI6 for a number of years and was working with the Spanish secret service investigating the Russian mafia shortly before his death.
He was said to regularly meet with an MI6 handler, named only as Martin, in central London and was paid by both the British and Spanish secret services into a joint bank account he held with his wife.
The hearing was adjourned ahead of a directions hearing on 14 March.
The coroner gave no indication on when he would conduct the private hearing but told the pre-inquest review that he could reconvene proceedings relating to the files in public if he deemed it possible to do so.