The system of so-called "shadow banking" blamed for aggravating the global financial crisis grew to $67 trillion globally last year, a new high, amid calls from the world's top policymakers for greater control of the sector.
A report by the Financial Stability Board (FSB) appeared to confirm fears among policy makers that shadow banking is set to thrive, beyond the reach of a regulatory net tightening around traditional banks and their activities.
Officials at the European Commission in Brussels see closer control of the sector as important in preventing a repeat of the financial crisis that toppled banks over the past five years and rocked the euro zone.
The study by the FSB, set up by the world's top economies (G20) to police global finance, 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 risen to $67 trillion in 2011, more than the total economic output of all the countries in the study.
The multi-trillion 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, and 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.
McCulley said 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 advocates 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 securitisation, 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 unravelled.
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.
