LONDON (Reuters) - A British coroner said on Wednesday he would hold an open and "fearless" inquiry into the murder in 2006 of former KGB agent Alexander Litvinenko after Britain 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 Russian leader Vladimir Putin, died after someone slipped polonium-210, a rare radioactive isotope, into his cup of tea at a plush London hotel.
At a pre-inquest hearing on Tuesday, lawyers for the British government argued 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 Britain was trying to hide details of his work for its MI6 intelligence service, and material which showed Russia was behind his death, because London 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 British Prime Minister David Cameron has tried to improve relations and strengthen business links since coming to power in 2010.
The coroner, High Court judge Robert 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.
He promised the PII request from British Foreign Secretary William Hague 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," Owen 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 this 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 told reporters. "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.
(Editing by Alistair Lyon)