The widow of poisoned spy Alexander Litvinenko has accused a UK coroner of abandoning "his search for the truth about Russian state responsibility for her husband's death".
Marina Litvinenko's fierce criticism comes after Robert Owen, the coroner presiding over her husband's inquest, revealed he cannot hear in public evidence linked to the alleged involvement of the Russian government.
The ruling was published after the coroner accepted an application by the UK Foreign Office to keep certain information under wraps.
A statement from Mrs Litvinenko's solicitors said: "This is a very sad day for Mrs Litvinenko, a tragedy for British justice which has until now been respected around the world, and it is frightening precedent for all of those, around the world, who have been trying so hard to expose the crimes committed by conspiracy of organised that operate from the Kremlin."
The statement from Mrs Litvinenko's solicitors went on: "Mrs Litvinenko is utterly dismayed by the Coroner's decision to abandon his search for the truth about Russian state responsibility for her husband's death.
"The effect of today's ruling is to protect those responsible for ordering the murder of a British citizen on the streets of London, and to allow the Russian government to shield behind a claim for secrecy made by William Hague with the backing of the Prime Minister David Cameron."
The statement added: "All those concerned with exposing the truth will be shocked and saddened that a political deal has been done between the two governments to prevent the truth from ever seeing the light of day."
Mr Litvinenko, 43, was poisoned with radioactive polonium-210 while drinking tea at the Millennium Hotel in London's Grosvenor Square in 2006.
The coroner's ruling means that the inquest into Mr Litvinenko's death could be scrapped and replaced with a public inquiry to allow evidence to be heard in secret.
His family believes he was working for MI6 at the time and was killed on the orders of the Kremlin.
The statement from Mrs Litvinenko's solicitors said they would now be pressing the coroner to order a full public inquiry as a matter of urgency as this is "the only way now that the truth can be exposed" despite meaning secret evidence will be considered in a closed session.
In his ruling, Mr Owen said: "It is my present view that I should hear submissions as to whether I should invite the Secretary of State (the Home Secretary) on behalf of Government to consider whether the power to hold an inquiry should be exercised in this case."
He said that the issues of preventability and Russian involvement are of "central importance" to the investigation into Mr Litvinenko's death.
Addressing the first issue, he said his duty to carry out "a full, fair and fearless investigation" would be hampered if it was not included.
He also said that excluding key evidence on the issue of Russian involvement would cause him "grave concern".
The coroner went on: "Were an inquiry to be held into the circumstances of Mr Litvinenko's death, the relevant material could be taken into account."
Alex Goldfarb, a friend of the Litvinenko family and his widow Marina, said the decision was "deeply dismaying", and that it was "upsetting" that in his view British government had prioritised its political relationship with Russia over the need to hold an open inquest.
He said: "It appears the British government is more concerned about the use of chemical weapons in Syria than radioactive weapons being used on the streets of London."
However Mr Goldfarb added: "On the other hand, it's an admission by the British government that the Russian state is culpable because otherwise they would not have requested immunity. That in itself is a partial victory for Marina."