The study, titled The Price of Offshore Revisited was released on Sunday by advocacy group Tax Justice Network, drew data from the World Bank, the International Monetary Fund and the United Nations, and the Bank for International Settlements.
Written by James Henry, a former chief economist at consultancy firm McKinsey, the study is thought to be the most detailed and rigorous study ever made of financial assets held in offshore financial centres and secrecy structures.
According to the study, at the end of 2010, the top 50 private banks collectively managed more than $12.1 trillion in cross-border invested assets for private clients, up from $5.4 trillion in 2005.
The study added:
While the numbers are shocking, Henry claims the $21 trillion figure is in fact “conservative” and the true value could be as high as $32 trillion.
According to Henry, these assets are "protected by a highly paid, industrious bevy of professional enablers in the private banking, legal, accounting and investment industries taking advantage of the increasingly borderless, frictionless global economy".
Calling the gap a “black hole” in the world economy, the money could generate tax revenues of between $190 to $280 billion, assuming a 30 percent tax rate, or approximately twice the amount OECD countries spend on annual overseas development assistance.
However, tax expert and British government adviser John Whiting said he was doubtful of the figure.
"There clearly are some significant amounts hidden away, but if it really is that size what is being done with it all?" he asked.