The widow of murdered spy Alexander Litvinenko said she does not believe a public inquiry into his death has been sparked by international tension with Russia.
Marina Litvinenko insisted that the probe would have happened anyway without pressures over Ukraine.
She said: "I've been waiting for this decision since February. I believed it would happen one day, and it's happened when it's a very difficult situation in the world.
"What happened in Ukraine has made my case a very different prospect, but I'm definitely sure (the decision) was not made because of this."
She said she has not fought for a public inquiry as a move against the UK or Russia, but for the truth.
"I've done this for justice, I've done this for truth. I would like to show people you are able to get justice in any difficult situation."
Former KGB bodyguard Andrei Lugovoi and Dmitri Kovtun have been identified as the prime suspects in the killing, but both deny any involvement and remain in Russia.
Mrs Litvinenko says she believes that they will never face extradition to the UK.
Referring to Russian president Vladimir Putin, she said: "I believe he will never change his mind."
But she said the public inquiry offered "a different type of justice."
Although Mrs Litvinenko and her lawyers will not be able to see secret material that will be part of the evidence in the inquiry, the chairman can take it into account, unlike in an inquest.
Solicitor Elena Tsirlina said: "The advantage of having a public inquiry is that the material that could not be disclosed as part of the inquest can now be seen by the chair of the inquiry and he will be able to take it into account, although that material will not be publicly disclosed."
Mrs Litvinenko said: "This is a different type of justice. For me it's very important, this type of justice, because there will still be consideration of why Sasha died."
S he fought for the probe into her husband's murder after a coroner said he could not hold a "fair and fearless" investigation as part of an inquest, and that a public inquiry should take place instead.
The 43-year-old Russian spy, known as Sasha to his loved ones, died after drinking tea laced with radioactive polonium-210 with two former colleagues at a London hotel in 2006.
Mrs Litvinenko said in a statement: "I am relieved and delighted with this decision. It sends a message to Sasha's murderers: no matter how strong and powerful you are, truth will win out in the end and you will be held accountable for your crimes.
"It has taken nearly eight years to bring those culpable for Sasha's murder to justice. I look forward to the day when the truth behind my husband's murder is revealed for the whole world to see."
Home Secretary Theresa May announced the probe in a written ministerial statement, saying: "I very much hope that this inquiry will be of some comfort to his widow."
The move means investigators can probe whether the Russian state was behind the murder, but it will not look at whether UK authorities could have prevented his death.
Previously the Government had resisted launching a public inquiry, and instead said it would "wait and see" what a judge-led inquest found.
But Mrs Litvinenko challenged this and the High Court ruled that the Home Secretary must reconsider the decision.
The move is likely to anger Mr Putin at a time when relations are strained in the aftermath of the downing of the Malaysia Airlines flight in Ukraine. Earlier David Cameron's official spokesman said there was "no link whatsoever" between the announcement of the inquiry and current tensions with Russia over its activities in Ukraine.
Sir Robert Owen, the senior judge who was acting as coroner in the inquest into Mr Litvinenko's death, will chair the public inquiry, which is set to begin on July 31 and finish at the end of next year. The terms of reference for the probe are "to conduct an investigation into the death of Alexander Litvinenko in order to ascertain who the deceased was; how, when and where he came by his death; identify where responsibility for the death lies and make appropriate recommendations".
Because there is no evidence to suggest that Mr Litvinenko "was or ought to have been assessed as being at a real and immediate threat to his life", the probe will not examine "the question of whether the UK authorities could or should have taken steps which would have prevented the death".
Mr Litvinenko, who fled to Britain in 2000, was poisoned while drinking tea with two Russian men, one a former KGB officer, at the Millennium Hotel in London's Grosvenor Square. His family believes he was working for MI6 at the time and was killed on the orders of the Kremlin.
Lugovoi and Kovtun have been identified as the prime suspects, but both deny involvement and remain in Russia.
Mrs May said in the statement: "The arrangements for the inquiry will now be a matter for Sir Robert Owen. I am very grateful to Sir Robert for continuing to lead the independent judicial investigation into Mr Litvinenko's death. It is more than seven years since Mr Litvinenko's death, and I very much hope that this inquiry will be of some comfort to his widow Mrs Litvinenko."
Downing Street said the inquiry will hold most of its hearings in public, though it might go into closed session to deal with material which could put national security at risk.
Under the Inquiries Act, Sir Robert will have the power to demand the production of witnesses and papers within UK jurisdiction, including agents and documents from the security and intelligence services. However, he has no such powers in relation to evidence from Russia.
Mr Cameron's spokesman said: "There will be provision within this inquiry for material that goes before it, if absolutely necessary, to be considered in closed session, where we believe and the judge accepts that this is in the national interest."
In a letter to Sir Robert last year, setting out the reasons for her opposition to an inquiry, Mrs May admitted relations with Russia were a factor in the decision.
She said: "It is true that international relations have been a factor in the Government's decision-making.
"An inquest managed and run by an independent coroner is more readily explainable to some of our foreign partners, and the integrity of the process more readily grasped, than an inquiry, established by the Government, under a chairman appointed by the Government, which has the power to see Government material potentially relevant to their interests, in secret.
"However, this has not been a decisive factor and, if it had stood alone, would not have led the Government to refuse an inquiry."