Former Russian Prime Minister Yegor Gaidar, who fell violently ill in Ireland two weeks ago, says he believes somebody tried to poison him.
Mr Gaidar was rushed to intensive care after collapsing in Dublin
He also said opponents of the Russian authorities were probably responsible.
Writing in the Financial Times, Mr Gaidar said he had "rejected the idea of complicity of the Russian leadership almost immediately".
He collapsed a day after the death from radiation poisoning of ex-KGB agent and Kremlin critic Alexander Litvinenko.
Mr Gaidar recalled that on 24 November, while attending a conference in Dublin, he suffered irresistible fatigue, then fell unconscious for several hours and bled heavily from the nose and throat.
"I remember my state before breakfast very well. It was excellent. Half an hour later it was awful."
Irish doctors did not find any evidence of radiation poisoning in his case, but said his health had suffered sudden "radical changes".
Mr Gaidar, a 50-year-old liberal economist, was transferred to a Moscow clinic. He said he had arranged to fly home because the Russian doctors knew his medical history.
But, 60 hours after he had fallen ill, it had been impossible for Russian doctors to determine the poison, he said, "especially if we are talking about secret toxic substances", traces of which usually disappear within 48 hours.
Mr Gaidar, who left hospital on Monday, heads the Institute for the Economy in Transition.
He said that if it was "attempted murder, politics was behind it," because he did not have any significant business interests to motivate a mafia-style hit.
Mr Gaidar served as Russia's prime minister under Boris Yeltsin following the break-up of the Soviet Union. He oversaw the lifting of price controls and was widely blamed by Russians who saw their savings devalued.
He has criticised some aspects of President Vladimir Putin's economic policy, but is not seen as a prominent political opponent of the Kremlin.
Critics of Mr Putin have blamed agents of the Russian state for the deaths of Litvinenko and another fierce critic of the Kremlin - journalist Anna Politkovskaya, who was shot dead in October.
Mr Gaidar said that in his own case it was "most likely" that "some obvious or hidden adversaries of the Russian authorities stand behind the scenes".
He blamed "those who are interested in further radical deterioration of relations between Russia and the West".
His comments also appeared in the Russian newspaper Vedomosti on Thursday.
Last week, his daughter said she suspected his illness was linked to the poisoning of Litvinenko.
She said some elements in the Russian security services and opponents of the government now living in exile abroad were intent on destabilising Russia.