A public inquiry will be held into the death of the Russian dissident Alexander Litvinenko, the UK Home Secretary Theresa May has announced.
Mr Litvinenko, a former KGB officer who became a British citizen, died in 2006 in a London hospital after he was poisoned with radioactive polonium.
The investigation will examine whether the Russian state was behind his death.
Mr Litvinenko's widow, Marina, said she was "relieved and delighted", saying the "truth will win out in the end".
Announcing the inquiry, Mrs May said she hoped it would be of "some comfort" to Mrs Litvinenko.
The inquiry will be chaired by senior judge Sir Robert Owen, who was the coroner at Mr Litvinenko's inquest last year.
Sir Robert delayed the inquest and called for a public inquiry because the inquest could not consider sensitive evidence because of national security fears.
That inquiry will now go ahead, with much of the evidence in public but some closed sessions for sensitive evidence.
Mr Litvinenko, 43, died after he was poisoned with radioactive polonium while drinking tea with two Russian men, one a former KGB officer, at a London hotel.
His family believes he was working for MI6 at the time and was killed on the orders of the Kremlin.
Speaking at a press conference, Mrs Litvinenko - who had legally challenged the government's earlier decision not to hold a public inquiry - said she had pursued the case "for justice", adding: "I did this for truth."
One of the suspects, Andrei Lugovoi, told the Russian Interfax news agency the decision to launch an inquiry was "the height of cynicism".
In May 2007, the UK said Mr Lugovoi - now a politician in Russia - should be charged with the murder of Mr Litvinenko. Russia refused to extradite Mr Lugovoi, who denies any involvement.
Analysis from BBC security correspondent Gordon Corera
Until now, the government has steadfastly resisted holding a public inquiry.
That was because there are layers of secrets surrounding the death of Alexander Litvinenko. This is thought to include secret intelligence that may relate to whether the Russian state was responsible for his murder.
There are also secrets about Mr Litvinenko's own relationship with MI6. The government demanded all these secrets be kept out of an inquest.
But the former Russian security officer's widow has fought a long legal battle to get to the truth.
A public inquiry will now look at where responsibility lies for the death although it does not look as if it will look at whether his relationship with MI6 means that more should be done to have protected him.
Lawyers for Mrs Litvinenko had claimed that the issue of state responsibility was being closed down precisely to try to improve relations with Russia.
If so, then changing times may explain a government's change of heart. And so we may get one step closer to finding out who was behind a radioactive murder on the streets of London.
The inquiry's remit will include finding out "where responsibility for the death lies" and making "appropriate recommendations".
But because there was no evidence before the death to suggest Mr Litvinenko was in danger, the inquiry would not examine whether UK authorities "could or should have taken steps" to protect him, the government said.
A Downing Street spokesman said Sir Robert would have the jurisdiction to demand the production of both witnesses - including security agents - and documents from the security and intelligence services.
But the spokesman said the inquiry, which is due to begin on 31 July and conclude by the end of 2015, would have no such powers in relation to evidence from Russia.
The Litvinenko case
- 1 Nov 2006 - Alexander Litvinenko has tea with former agents Andrei Lugovoi and Dmitri Kovtun in London
- 4 Nov 2006 - After three days of vomiting he is admitted to hospital, and dies 22 days later. His death is attributed to radiation poisoning
- May 2007 - The UK decides Mr Lugovoi should be charged with the murder of Mr Litvinenko. He denies any involvement but says Mr Litvinenko was a British spy
- 5 Jul 2007 - Russia officially refuses to extradite Mr Lugovoi, prompting a diplomatic row
- 20 Sept 2012 - Pre-inquest review hears that Russia's links to the death will be probed
- May-June 2013 - Inquest into Mr Litvinenko's death delayed as coroner decides a public inquiry would be preferable
- Jan 2014 - Marina Litvinenko in High Court fight to force a public inquiry
- 11 Feb 2014 - High Court says the Home Office had been wrong to rule out an inquiry before the outcome of an inquest
Former director of public prosecutions Ken Macdonald said: "This was a particularly foul murder; the infliction of a slow, lingering radioactive death."
He said Mr Litvinenko was "under the protection" of Britain at the time, and if Russia was involved the inquiry would "expose that".
BBC political editor Nick Robinson said Whitehall sources had told him the timing of the announcement - coming at the same time as the fallout from the Malaysia Airlines crash in Ukraine - was "a coincidence".
Western leaders have accused Russia of arming rebels in eastern Ukraine, who they believe shot down flight MH17 with a ground-to-air missile.