Citing sources inside the president's administration, Belkovsky says Putin owns vast holdings in three Russian oil and gas companies, concealed behind a "non-transparent network of offshore trusts".
Putin "effectively" controls 37% of the shares of Surgutneftegaz (SGTZY), an oil exploration company and Russia's third biggest oil producer, worth $20 billion, Belkovsky says. He also owns 4.5% of OAO Gazprom (GAZP.RS), and "at least 75%" of Gunvor, a mysterious Swiss-based oil trader, Belkovsky alleges.
Putin hasn't commented on Belkovsky's allegations. The Guardian put the allegations to the Kremlin but was told Putin's chief spokesman, Dmitry Peskov, wasn't available.
Belkovsky - who published a book about Putin's finances last year, and who is the director of the National Strategic Institute, a Moscow thinktank - says he is confident of his assessment of Putin's hidden wealth.
"It's not a secret among the elites," he said. "But please pay attention that Vladimir Vladimirovich [Putin] has never sued me."
In an interview Wednesday with Time magazine, which named Putin its person of the year, the president vehemently denied those inside the Kremlin were corrupt.
Asked whether "some of the people closest to you are getting rich," Putin said: "Then you know who and how. Write to us, to the foreign ministry, if you are so confident. I presume you know the names, you know the systems and the tools.