"The argument that financial institutions do not need the new rules to help them avoid the irresponsible actions that led to the crisis of 2008," said U.S. Rep. Barney Frank (D., Mass.), "is at least $2 billion harder to make today."
Frank, the retiring Democratic leader of the House Financial Services Committee, said in a statement that the revelation runs counter to JPMorgan's narrative "blaming excessive regulation for the woes of financial institutions."
Dimon has been among Wall Street's most outspoken critics of efforts to regulate the financial industry more heavily.
Cliff Rossi, a former top risk executive for Citigroup, Countrywide, and other big financial companies, said he drew little hope from the steps Washington has taken.
He said JPMorgan's loss showed that the market for the complex financial instruments known as derivatives was too opaque. He also said the loss demonstrated that banks like JPMorgan were too big to manage effectively.
"This just tells you that we are a long, long way from getting our arms around this whole 'too big to fail' issue," said Rossi, now executive-in-residence at the University of Maryland's business school.
"This is actually worse than Citigroup landing in trouble" in 2008, he said, "because JPMorgan is recognized as one of the better-run institutions."
The head of the Securities and Exchange Commission, Mary Schapiro, told reporters that the agency was focused on the JPMorgan loss, but declined to comment further.
Experts and industry officials were skeptical that the trading was designed to protect against JPMorgan's own losses, as Dimon contended late Thursday in a surprise conference call with stock analysts and reporters.
The bank appeared to have been betting for its own benefit, a practice known as "proprietary trading," said Rossi and other former bank executives.
Dimon said the type of trading that led to the $2 billion loss would not be banned by the so-called Volcker rule, which was still being written and was expected to ban certain types of trading by banks with their own money.
The Federal Reserve said last month that it would begin enforcing that rule in July 2014. Bank executives, including Dimon, have argued for weaker rules and broader exemptions.
JPMorgan has been a strong critic of provisions that would have made this loss less likely, said Michael Greenberger, former enforcement director of the Commodity Futures Trading Commission, which regulates some derivatives.
"These instruments are not regularly and efficiently priced, and a company can wake up one day, as AIG did in 2008, and find out they're in a terrific hole. It can just blow up overnight," said Greenberger, a professor at the University of Maryland.
On Friday, bank stocks were hammered in Britain and the United States, partly because of fear that the JPMorgan loss would lead to tougher regulation of financial institutions.
JPMorgan stock was the hardest hit, but its American counterparts suffered, too: Citigroup and Morgan Stanley fell 4.2 percent, and Goldman Sachs dropped 3.9 percent.
Stock analysts said that bank stocks were hurt mostly because of regulatory fear, not because there was reason to believe other banks would discover similar losses.
"The regulatory and political environment is already a headwind," Deutsche Bank said in a note to clients, "and clearly this doesn't help."