It said in a report that up to USD 2 trillion (1.9 trillion euros) was laundered each year, often through complex webs of anonymous entities across multiple jurisdictions, and said the Group of 20 top industrial economies must do more to eradicate the legal structures that made it possible.
The report was issued shortly before a G20 summit in Turkey, a year on from their gathering in Australia where leaders committed to improving transparency to help prevent the use of shell companies for illegal purposes.
"Pick any major corruption scandal in recent history -- Petrobras, FIFA, Ukraine's Viktor Yanukovych -- and you will find a secret company was used to pay a bribe, shift and hide stolen money, or buy luxury real estate in places like London and New York," said Transparency's managing director Cobus de Swardt.
Anti-graft advocates have repeatedly called for the adoption of tougher corporate disclosure requirements by G20 countries, which represent 85 percent of the global economy.
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"It makes no sense that this gaping loophole for the corrupt remains open," de Swardt said in a statement.
Transparency called for governments to set up a central public register containing information on beneficial ownership and to tighten up corporate oversight on those aiding the corrupt.
Currently, it said, only Britain and India now require companies to record information about their real owners, while Brazil and South Africa had not even adopted a legal definition of beneficial ownership.
And it said that properties worth hundreds of billions of dollars in London and New York have secret owners, because in seven G20 nations estate agents are not required to identify the true buyers and sellers.