Russian doctors said they had identified no natural cause for an ailment afflicting Yegor Gaidar, architect of Russia's market reforms, Gaidar's spokesman said on Thursday (November 30). "According to preliminary information from doctors, they do not see at the moment a natural reason for the poisoning," Gaidar's spokesman Valery Natarov told Reuters. "They think it is a substance they cannot so far identify -- it is not a natural poisoning," he said. "It is too early to say whether it is a poison or not." His daughter, Maria Gaidar, told Ekho Moskvy radio station that Gaidar was feeling much better and was still in hospital. Gaidar, 50, a former acting prime minister who is now an influential academic, was taken to hospital earlier this week after he collapsed during a visit to Ireland to present his new book. He was later moved to a Moscow hospital. Gaidar has largely quit politics and now focuses on research in his Moscow-based Institute for the Economy in Transition. His illness followed the death in London of a strong critic of President Vladimir Putin, ex-spy Alexander Litvinenko, who was poisoned with a radioactive substance. The Kremlin has denied any link to the death of Litvinenko, who accused Putin of killing him. Another prominent critic of Putin, journalist Anna Politkovskaya, was shot dead in Moscow in October. No one has been detained in connection with the killing. Gaidar's former associate Anatoly Chubais, who now heads Russia's electricity grid, has suggested Gaidar could have been poisoned but did not say who could have been responsible. The Kremlin said on Thursday Putin had telephoned Gaidar to wish him a speedy recovery.