The widow of former KGB spy Alexander Litvinenko who was killed in London has moved a step closer to a public inquiry into her husband's death.
The High Court said the Home Office had been wrong to rule out an inquiry before the outcome of an inquest.
Marina Litvinenko said only a public inquiry would uncover the Russian state's alleged role in her husband's 2006 death from radiation poisoning.
The Home Office said it would carefully consider the judgement.
'No rational basis'
Speaking outside court, Mrs Litvinenko said she was "very glad" and that Home Secretary Theresa May should accept the court's decision.
Last year a coroner ruled a public inquiry would be better placed to establish how Mr Litvinenko died - even taking into account restrictions on what material could be disclosed on national security grounds.
But ministers refused to launch an inquiry, which could have wider powers than an inquest, saying that they wanted to wait and see whether an inquest would be able to establish the facts.
Mrs Litvinenko challenged that decision saying that the home secretary's refusal to hold a public inquiry was irrational, given the serious allegations at the heart of the death.
In a ruling delivered by three senior judges, the High Court said Mrs May's decision had been wrong and she should think again.
Lord Justice Richards said: "I am satisfied that 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.
"The deficiencies in the reasons [given by the home secretary] are so substantial that the decision cannot stand. The case for setting up an immediate statutory inquiry as requested by the coroner is plainly a strong one.
"The existence of important factors in its favour is acknowledged, as I have said, in the secretary of state's own decision letter."
The three judges stopped short of saying that it would be irrational to refuse a public inquiry in all circumstances - but they said that the home secretary would have to come up with legally "better reasons" than those she had given to Mrs Litvinenko.
Mrs Litvinenko said: "I am delighted with today's decision. It is a real milestone in my fight to get to the truth behind my husband's murder.
"We have always said that the reasons that the government gave for preventing a full inquiry into the involvement of the Russian state didn't make sense. Now the High Court has agreed with us. Every one of the six reasons given by the government were found to be illogical."
A Home Office spokesperson said: "We are carefully considering the judgement. The government continues to fully co-operate with the coroner's inquest into Mr Litvinenko's death."
During the judicial review, Ben Emmerson QC, for Mrs Litvinenko, argued that the home secretary had adopted a "bizarre" position by backing an inquest that would be so restricted by national security considerations it could not examine secret intelligence allegedly linking Moscow to the death.
He said that there was "a strong and overwhelming" need for a public inquiry to establish whether the former spy was killed for some private criminal reason or whether he was the victim of "state-sponsored assassination".