A liberal advocacy organization opposed to tax avoidance has released a report estimating the world's wealthiest individuals have stashed at least $21 trillion in offshore tax havens: an amount greater than the combined GDP of the United States and Japan.

The London-based Tax Justice Network calls its current estimate "conservative," and says the true figure could potentially be as high as $32 trillion, more than twice the U.S. GDP of $15 trillion. The report updates a 2005 study that then put the offshore total at $11.5 trillion.

The report, prepared by by former McKinsey & Co. Chief Economist James Henry and released this week, acknowledges the difficulty in estimating numbers that are intentionally hidden by the "rich and powerful." Consequently, it supplements data from the IMF, UN and central banks with its own data mining to indirectly establish what it considers a "base case," or safe estimate.