A global super-rich elite had at least $21 trillion (£13tn) hidden in secret tax havens by the end of 2010, according to a major study.
The figure is equivalent to the size of the US and Japanese economies combined.
The Price of Offshore Revisited was written by James Henry, a former chief economist at the consultancy McKinsey, for the Tax Justice Network.
Tax expert and UK government adviser John Whiting said he was sceptical that the amount hidden was so large.
Mr Whiting, director of the Office of Tax Simplification, said: "There clearly are some significant amounts hidden away, but if it really is that size what is being done with it all?"
Mr Henry said his $21tn is actually a conservative figure and the true scale could be $32tn. A trillion is 1,000 billion.
Mr Henry used data from the Bank of International Settlements, International Monetary Fund, World Bank, and national governments.
His study deals only with financial wealth deposited in bank and investment accounts, and not other assets such as property and yachts.
The report comes amid growing public and political concern about tax avoidance and evasion. Some authorities, including in Germany, have even paid for information on alleged tax evaders stolen from banks.
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