The system of so-called "shadow banking," blamed by some for aggravating the global financial crisis, grew to a new high of $67 trillion globally last year, a top regulatory group said, calling for tighter control of the sector.
A report by the Financial Stability Board (FSB) on Sunday appeared to confirm fears among policymakers that shadow banking is set to thrive, beyond the reach of a regulatory net tightening around traditional banks and banking activities.
The FSB, a task force from the world's top 20 economies, also called for greater regulatory control of shadow banking.
"The FSB is of the view that the authorities' approach to shadow banking has to be a targeted one," the group wrote in a report, noting the current lax regulation of the sector.
"The objective is to ensure that shadow banking is subject to appropriate oversight and regulation to address bank-like risks to financial stability," it said.
Officials at the European Commission in Brussels also see closer oversight of the sector as important in preventing a repeat of the financial crisis that has toppled banks over the past five years and rocked the euro zone.
The study by the FSB said shadow banking around the world more than doubled to $62 trillion in the five years to 2007 before the crisis struck.
But the size of the total system had grown to $67 trillion in 2011 - more than the total economic output of all the countries in the study.
The multitrillion-dollar activities of hedge funds and private equity companies are often cited as examples of shadow banking.
But the term also covers investment funds, money market funds and even cash-rich firms that lend government bonds to banks, which in turn use them as security when taking credit from the European Central Bank.
Even the man credited with coining the term, former investment executive Paul McCulley, gave a catch-all definition, saying he understood shadow banking to mean "the whole alphabet soup of levered up non-bank investment conduits, vehicles and structures," such as the special investment vehicles that many blamed for the financial crisis.
The United States had the largest shadow banking system, said the FSB, with assets of $23 trillion in 2011, followed by the euro area - with $22 trillion - and the United Kingdom - at $9 trillion.
The U.S. share of the global shadow banking system has declined in recent years, the FSB said, while the shares of the United Kingdom and the euro area have increased.
The FSB warned that tighter rules that force banks to hoard more capital reserves to cover losses could bolster shadow banking.
It advocated better controls, although cautions that shadow banking reforms should be dealt with carefully because the sector can also be a source of credit for business and consumers.
Forms of shadow banking can include securitization, which can transform bank loans into a tradeable instrument that can then be used to refinance credit, making it easier to lend.
In the run-up to the crisis, however, banks such as Germany's IKB stored billions of euros of such instruments in off-balance sheet vehicles, which later unraveled.
Another example is a repurchasing agreement, or repo, where a player such as a hedge fund could sell government bonds it owns to a bank, agreeing to repurchase them later.
The bank may then lend those bonds onto another hedge fund, taking a position on the government debt. Such agreements are used by banks to lend and borrow. A risk could arise if one of the parties in the chain collapses.
The European Commission is expected to propose EU-wide rules for shadow banking next year.