A British coroner has said he will hold an open and "fearless" inquiry into the 2006 murder of Alexander Litvinenko, former KGB agent.
Wednesday's statement in a London court by Robert Owen, high court judge, came after the UK government asked for sensitive information about the death to be kept secret.
Litvinenko, 43, who had been granted British citizenship and had become a vocal critic of Vladimir Putin, died after ingesting polonium-210, a rare radioactive isotope, in his tea at a hotel in the British capital.
At a pre-inquest hearing on Tuesday, lawyers for the British government argued that information it held should be subject to a public interest immunity (PII) certificate, barring disclosure which they said would seriously harm national security.
The lawyer for Litvinenko's family argued that Britain was trying to hide details of his work for its MI6 intelligence service, and material which showed Russia was behind his death, because the UK wanted to protect lucrative Russian trade deals.
Ties between Britain and Russia fell to a post-Cold War low in the immediate aftermath of Litvinenko's death, but David Cameron, UK prime minister, has tried to improve relations and strengthen business links since coming to power in 2010.
Owen ruled he would go ahead with private hearings to consider the government's request, but said he would only allow material to be kept secret where that served the public interest better than disclosure.
Owen promised the PII request from William Hague, British foreign secretary, would be "subjected to the most stringent and critical examination".
"It is my duty to carry out a full, fearless and independent investigation into the circumstances of the death of Mr
Litvinenko. That, I intend to do," Owen told Wednesday's hearing at London's Royal Courts of Justice.
"[The inquest] will be conducted with the greatest possible degree of openness and transparency," he said.
Under British law, inquests conducted by coroners are held when a person dies unexpectedly to determine the cause of death.
Owen said he would go ahead with closed hearings next week when the government will put forward its case why certain material should be withheld from the inquest.
Anything that is subject to PII would be excluded from evidence and would not form part of Owen's final judgement.
Ben Emmerson, the lawyer for Litvinenko's widow Marina, said on Tuesday this could result in a situation where Owen could be shown secret evidence which conclusively showed the Kremlin was complicit in murder, but would then have to issue a public judgment which exonerated Russia of any involvement.
A previous hearing has already been told the British government possessed information which established "a prima
facie case" that the Russian state was behind the killing.
British police and prosecutors have also said there was enough evidence to charge two former KGB agents, Andrei Lugovoy and Dmitry Kovtun, with murder.
Lugovoy denies any involvement and Moscow has repeatedly dismissed allegations it ordered Litvinenko's death to silence him.
Marina Litvinenko said although Owen's decision was not ideal, she had faith in his commitment that crucial facts about her husband's death would not be covered up.
"I believe he will do exactly what he said to me," she said.
"I do trust him. I believe he is still in the same position that if he sees evidence the Russian state is behind the crime he will use it."
Owen said the next hearing to decide when the full inquest will start would take place on March 14.