JPMorgan Chase & Co. Chief Executive Officer Jamie Dimon conceded a key point when pressed by lawmakers about a proposed ban on proprietary trading at banks: Had the rule been in place, it may have prevented the firm's recent $2 billion loss.

The ban “may very well have stopped parts of what this portfolio morphed into,” Dimon said yesterday in testimony to the Senate Banking Committee.

Dimon's comments provided new ammunition to lawmakers and regulators emboldened by JPMorgan's mistakes who argue that a stricter ban on banks using their own money to make trades is needed to prevent a repeat of the 2008 financial crisis. The ban, part of the 2010 Dodd-Frank Act, was named for Paul Volcker, the former Fed chairman who championed the measure.