It was billed by Barack Obama as the toughest crackdown on Wall Street since the great depression. But top US banks could be given until 2022 to comply with the so-called Volcker rule, which is supposed to restrict financial institutions' risker trading activities.

A string of delays and extension periods written into a final version of Congress's financial regulation reform bill means that firms such as Citigroup and Goldman Sachs could exploit loopholes until 2022 before withdrawing from "illiquid" funds such as private equity. The long gestation period is an example of the degree of compromise inserted into the package following months of lobbying on Capitol Hill by powerful banks.

"You can't just say 'stop', you can't just say 'unwind,'" said Lawrence Kaplan, a lawyer at Paul, Hastings, Janofsky & Walker in Washington, who said the delay was a dose of political reality. "These things have contracts and detailed legal frameworks. You can't undo them without doing considerable harm."

The Volcker rule, championed by formed Federal Reserve boss Paul Volcker, stops banks from engaging in "proprietary trading" whereby they trade with their own capital, rather than clients' money. It also severely restricts their investments in high-risk hedge funds and private equity ventures.

Language in the act, according to Bloomberg News, allows for a six-month study and a further nine months of rule-making. The measure is supposed to become effective 12 months after the final rule is laid, then banks have two years to conform. But if they need to, they can apply for a three-year extension. On top of that, a five-year moratorium is available for "illiquid" funds that are hard to unwind.

Complicated caveats in the bill are subject to interpretation. A spokesman for Jeff Merkley, a Democrat who proposed various changes to the rule, told Bloomberg that the maximum delay was supposed to be nine years.

Other measures in Obama's reforms include the creation of a consumer protection agency, the introduction of a vote by shareholders' on boardroom pay and new powers for authorities to seize troubled financial institutions.

For Wall Street, the Volcker rule and curbs on derivatives trading are the most contentious changes. In a research note, analyst Jason Goldberg of Barclays Capital said JP Morgan, Bank of America and Citigroup would be most affected by a ban on proprietary trading. Taken together with the rest of the regulatory reform bill, Goldberg estimated that Obama's crackdown could cut earnings at 26 leading banks by 14% in 2013, eliminating nearly $18bn of profit.