A judge has overturned a decision by a coroner to release secret documents relating to the death of a former KGB agent.
Coroner Sir Robert Owen decided to partially lift a public interest immunity certificate during the inquest of Alexander Litvinenko.
Owens said the decision was necessary for him to be able to conduct a “fair and meaningful” hearing.
However, this decision was appealed by Foreign Secretary William Hague whose legal team warned the documents were “sensitive to the highest degree”.
The Government legal team said disclosure would damage the public interest.
Lord Justice Goldring, sitting with two other judges, declared the material must remain secret.
Quashing the coroner's decision to reveal it, the judge said: "I am driven to the conclusion that the weight the coroner gave to the views of the Secretary of State was insufficient and amounted to an error of law."
The judge, sitting with Lord Justice Treacy and Justice Mitting, said the issues raised by the case "concerned the risk of significant damage to national security" and added: "Nothing we have decided reduces the importance of open justice."
The coroner is considering whether to appeal.
Litvinenko, 43, a Russian dissident and former KGB agent, was poisoned in 2006 by radioactive polonium-210 while drinking tea during a meeting with former security colleagues at the Millennium Hotel in Grosvenor Square, London. He died three weeks later.
The death has raised questions over whether the Russian state was involved, as alleged by Litvinenko's widow Marina and his son Anatoly.
Mrs Litvinenko said through her QC she was "disappointed but not surprised" by today's ruling and she was "gathering her strength" for what might be her last attempt "to see the truth emerge about the Russian state's responsibility" for her husband's death.
The attempt will take the form of another application for judicial review – this time brought by her, challenging the Government refusal to grant the coroner powers "to take account of secret closed material and convert the inquest into a public inquiry".
Ben Emmerson QC, appearing for Mrs Litvinenko, told the judges that every stage of the legal process so far had left her "with increasing concern that the truth of the position about the Russian state's responsibility may not in due course be established by these proceedings".
The widow took "considerable comfort" from the fact that the closed material had been considered "by three senior judges wholly independent of the Government" who had put their conclusions in an open judgment for all to see.
The coroner agreed there should be secrecy in relation to the role of the Russian state and whether the UK could have done anything to prevent the death.
He also agreed to exclude any material relating to whether the British security and intelligence services could have prevented Litvinenko's death from radioactive poisoning.
But he refused to uphold the whole of the PII certificate signed by Hague relating to other material said to be "under the direct scope of the PII certificate".
Later a spokesman for the coroner said he would"reflect carefully" on the court's PII decision.
The spokesman said: "He will hear submissions on Friday on the scope of the inquest in the light of his May 17 ruling on PII and the judgment this morning.
"In due course he will make a ruling.
"Sir Robert will not be making any further comment on the JR decision before the hearing on Friday."