The British government has said there will be a public inquiry into the death of former Russian spy Alexander Litvinenko.
Mr Litvinenko was poisoned with radioactive polonium-210 while drinking tea at the Millennium Hotel in London's Grosvenor Square in 2006.
In May last year, the coroner presiding over the inquest into the 43-year-old's death revealed he could not hear in public evidence linked to the alleged involvement of the Russian government.
The ruling was published after the coroner accepted an application by the UK Foreign Office to keep certain information under wraps.
Mr Litvinenko's wife, Marina, accused the coroner at the time of abandoning "his search for the truth about Russian state responsibility for her husband's death".
Mr Litvinenko's family believes he was working for MI6 and was killed on the orders of the Kremlin.
Home Secretary Theresa May confirmed the decision in a written ministerial statement today, saying: "I very much hope that this inquiry will be of some comfort to his widow."
The latest move will mean investigators can examine whether the Russian state was behind his murder.
The government has until now resisted launching a public inquiry, and instead said it would "wait and see" what a judge-led inquest found.
But Mr Litvinenko's widow challenged this and the High Court ruled the Home Secretary must reconsider the decision.
The move is likely to anger Russian President Vladimir Putin at a time when relations are strained in the aftermath of the downing of the Malaysia Airlines flight in Ukraine.
Former KGB bodyguard Andrei Lugovoi and Dmitri Kovtun have been identified as the prime suspects, but both deny any involvement and remain in Russia.
Ministers have been under pressure since last year when Robert Owen, who was conducting the inquest, said he could not hold a "fair and fearless" investigation.
Ms Litvinenko said she is "relieved and delighted" that a public inquiry is to be held, and that it sends a message to her husband's killers that "no matter how strong and powerful you are, truth will win out in the end".