A Government Accountability Office report Wednesday said proprietary trading has produced a relatively small share of revenues at the six largest banks, but the study quickly came under fire from two senators who called the GAO's findings thin.

The financial reform law passed last year banned proprietary trading, and restricted other risky investments, at banks.

The provision known as the "Volcker Rule" — after former Federal Reserve Board Chairman Paul Volcker, who inspired the ban — also required the GAO to study aspects of such trading at big banks.

The report said regulators need to gather more information on the activities covered by the new rules.

It concluded that "standalone" proprietary trading at the six largest bank holding companies was about 3.1% of their combined quarterly revenue from June 2006 to December 2010.

"Compared to these firms' overall revenues, their stand-alone proprietary trading generally produced small revenues in most quarters and some larger losses during the financial crisis," the GAO said.

But Democratic Sens. Carl Levin and Jeff Merkley, who authored the trading provision in the Dodd-Frank Act, blasted the report as insufficient.

They said the accountability office, which claimed it only had information to study the banks' standalone trading operations, should have delved deeper to "shine more light on the high-risk gambles that decimated millions of families and businesses and nearly destroyed our financial system."

"GAO claims they could not access the data necessary to undertake the full study required by Congress. This is unfortunate. While we are not surprised that, even with its limited review, the GAO concluded that proprietary trading is riskier than other activities, a complete study of proprietary trading would have been of much greater value to policy makers and regulators," they said.

"The report reminds us of the story of a man who dropped his keys at night and then began looking for them under a nearby parking lot light, not because he dropped them there but because that was where the light was."

Subscribe Now

Access to authoritative analysis and perspective and our data-driven report series.

14-Day Free Trial

No credit card required. Complete access to articles, breaking news and industry data.