The coroner investigating Russian spy Alexander Litvinenko's death said there was a prima facie case that the Russian state was involved, it has emerged during a High Court hearing.
Lawyers for Mr Litvinenko's widow are applying for judicial review of the home secretary's "legally irrational" refusal to order a public inquiry.
Government lawyers, though, argue the decision was rational and lawful.
Mr Litvinenko, 43, was poisoned with radioactive polonium in London in 2006.
He had been an officer with the KGB before being granted asylum in the UK from where he criticised the Russian government.
His family say he was working for MI6, and killed on Kremlin orders.
Just over seven years ago, Mr Litvinenko was poisoned while drinking tea with two Russian ex-KGB officers, at the Millennium Hotel in Grosvenor Square, central London.
His death - and the allegation in the aftermath that Moscow was involved - caused considerable damage to the UK's relations with Russia.
There has since been a long-running legal fight between his widow Marina Litvinenko, the proposed coroner Sir Robert Owen and ministers over what should or should not be made public about the nature of Mr Litvinenko's death.
Last October, the government won a court order blocking the release of certain secret information at the proposed inquest.
At this latest hearing at the Royal Courts of Justice, papers just disclosed show that Sir Robert concluded that documents held by the UK government "establish a prima facie case as to the culpability of the Russian state in the death of Alexander Litvinenko".
The documents also showed that the coroner believed there was no evidence that the British authorities could have prevented his death, the court heard.
In a previously secret ruling, Sir Robert said there was no material in the documents to suggest that Mr Litvinenko's life should have been assessed as being under a "real and immediate" threat.
Mrs Litvinenko says she wants to get to "the truth" of how her husband died and believes the UK government was wrong not to order a public inquiry into his death.
Home Secretary Theresa May decided that a normal inquest should go ahead before deciding whether there should be a wide-ranging inquiry.
This was despite a request for an inquiry from Sir Robert, who argued an inquest could not properly investigate the alleged involvement of Moscow in Mr Litvinenko's death.
In a letter to the coroner explaining her decision, Mrs May said an inquest would be "more readily explainable to some of our foreign partners".
At Tuesday's court hearing, Ben Emmerson QC, for Mrs Litvinenko, argued there was a "strong and overwhelming" need for a public inquiry.
It was necessary in order to establish whether Mr Litvinenko was the victim of a crime committed "for private criminal purposes" or whether it was a "state-sponsored assassination carried out on the territory of the UK on the orders of the Russian government", he said.
He also accused Mrs May of adopting the "bizarre" position of waiting to see the outcome of an inquest that was so restricted it would be unable to examine secret evidence linked to allegations of Moscow's involvement.
Neil Garnham QC, for the home secretary, is arguing the claim should fail because the obligation to hold an effective, official investigation into the death is being discharged by the government through a police investigation and the proposed inquest.
The case is being heard over two days by Lord Justice Richards, Lord Justice Treacy and Mr Justice Mitting.
Mrs Litvinenko has appealed to the British public for money to help fund her fight for an inquiry.
She says she could face a bill of up to £40,000 if she loses. So far, she has received almost £15,000 in donations.