The world's super-rich had at least $21trn hidden in secret tax havens by the end of 2010, according to a new study.
The figure is equivalent to the size of the US and Japanese economies combined, said The Price of Offshore Revisited published on Sunday by the Tax Justice Network.
The report was written by James Henry, a former chief economist at the consultancy McKinsey.
Henry said the $21trn was actually a conservative figure and the true scale could be $32trn.
Henry said he used data from the Bank of International Settlements, International Monetary Fund, World Bank, and national governments.
The report said the number of the global super-rich who have amassed a $21trn offshore fortune is fewer than 10 million people.
Of these, less than 100,000 people worldwide own $9.8trn of wealth held offshore, it added.
Henry said: "The lost tax revenues implied by our estimates is huge. It is large enough to make a significant difference to the finances of many countries.
"From another angle, this study is really good news. The world has just located a huge pile of financial wealth that might be called upon to contribute to the solution of our most pressing global problems."