Marina Litvinenko challenged the UK Government's decision to await the outcome of an inquest before deciding if there should be an inquiry to probe more deeply into the killing.
Now three High Court judges have decided Home Secretary Theresa May must reconsider the decision in the light of their ruling. The judges gave the Government until Friday to decide whether to appeal.
Speaking for the three judges, Lord Justice Richards said: "Taking everything together, I am satisfied the reasons given by the Secretary of State do not provide a rational basis for the decision not to set up a statutory inquiry at this time but to adopt a 'wait and see' approach."
Mrs Litvinenko said she was "very glad" the ruling had gone in her favour.
Mrs Litvinenko wants to get to the truth of how her 43-year-old husband came to die in 2006 after fleeing Russia and receiving political asylum in the UK.
He was poisoned with radioactive polonium-210 while drinking tea with two Russian men, one a former KGB officer, at a London hotel. His family believes he was killed on the orders of the Kremlin.