A coroner in Britain said on Wednesday he would hold an open inquiry into the murder in 2006 of former KGB agent Alexander Litvinenko.
Litvinenko, 43, who had obtained British citizenship and was a vocal critic of Russian leader Vladimir Putin, died after drinking polonium-210, a rare radioactive isotope, into a cup of tea at a hotel in London.
At a pre-inquest hearing on Tuesday, government lawyers argued that parts of the hearing should be subject to a public interest immunity certificate, barring disclosure of documents which would seriously harm national security.
Litvinenko's family lawyer argued Britain was trying to hide details of his work for MI6 and material that showed Russia was behind his death, because London wanted to protect trade deals with Russia.
High Court judge Robert Owen ruled he would go ahead with private hearings to consider the government's request, but said he would allow material to be kept secret only where that served the public interest better than disclosure.
"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," Judge Owen told the hearing at the Royal Courts of Justice in London.
"(The inquest) will be conducted with the greatest possible degree of openness and transparency," he said.