Former Russian Prime Minister Yegor Gaidar was released from a Moscow hospital Monday evening following a mysterious illness, his spokesman said.

Doctors could give a final diagnosis of what struck the 50-year-old economist as early as Tuesday morning, spokesman Valery Natarov told The Associated Press.

Gaidar -- who served briefly as prime minister in the 1990s under President Boris Yeltsin and is a leader of a Russian liberal opposition party -- began vomiting and fainted during a conference in Ireland on Nov. 24, and was rushed into intensive care at a hospital.

He fell ill a day after ex-KGB officer and Kremlin critic Alexander Litvinenko died in London after being poisoned with the radioactive element polonium-210.

Gaidar's aides said doctors treating him in Moscow suspected he had been poisoned were working to determine how that might have happened. Irish doctors concluded he was not poisoned by a radioactive substance, but said his health had suffered sudden "radical changes."

Gaidar's illness added to growing speculation in Moscow over Litvinenko's death and who might be responsible.

Click here to go to FOXNews.com's Europe Center.