Mario Tama/Getty Images Jamie Dimon, the chief executive of JPMorgan Chase.

JPMorgan Chase. which emerged from the financial crisis as the nation’s biggest bank, disclosed on Thursday that it had lost more than $2 billion in trading, a surprising stumble that promises to escalate the debate over whether regulations need to rein in trading by banks.

Jamie Dimon. the chief executive of JPMorgan, blamed “errors, sloppiness and bad judgment” for the loss, which stemmed from a hedging strategy that backfired.

The trading in that hedge roiled markets a month ago, when rumors started circulating of a JPMorgan trader in London whose bets were so big that he was nicknamed “the London Whale” and “Voldemort,” after the Harry Potter villain.

For a bank that earned nearly $19 billion last year, the trading loss, which could go higher, will not cripple it in any way. Still it demonstrates how a market blunder can shake even a financial giant that celebrates its “fortress balance sheet.”

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It is a rare blow to the reputation of Mr. Dimon, 56, a native New Yorker known for his hands-on management style and a confident swagger. After successfully steering his bank through the market turmoil of 2008 and the recession, he is perhaps the most influential bank executive in the country — and a vocal critic of the efforts to write rules under the Dodd-Frank regulatory overhaul.

The setback for JPMorgan may strengthen the hand of regulators in Washington who are now writing the rules for Dodd-Frank — in particular the Volcker Rule. which restricts banks from trading with their own money.

JPMorgan’s setback “casts doubt on Jamie’s opposition and adds fuel to anyone who has been pushing for greater regulation,” said Mike Mayo, an analyst with Credit Agricole Securities. “Oh, how the mighty have fallen.”

Throughout the debate around the Volcker Rule, which is named after Paul A. Volcker. the former Federal Reserve chairman, Mr. Dimon has said the rule goes too far. Mr. Dimon told CNBC earlier this year that under the Volcker Rule, “if you want to be trading, you have to have a lawyer and a psychiatrist sitting next to you determining what was your intent every time you did something.”

Questions over whether banks have been engaging in such trading for themselves while calling it “market-making” or “hedging” came into focus last month, when the reports emerged that a trading unit of JPMorgan in London was taking such large positions in the name of hedging that they were distorting the market.

At the time, Mr. Dimon played down concerns about that trading, in what the bank calls its chief investment office, telling analysts in an April 13 conference call that it was “a complete tempest in a teapot.”

In a hastily organized conference call with analysts on Thursday, Mr. Dimon sounded more humble, saying that “egregious mistakes” were made. “They were self-inflicted and this is not how we want to run a business.”

Yet while conceding that the bank had “egg on its face,” Mr. Dimon refused to concede that the losses necessitated a stronger regulatory framework. Although he did says that it “plays right into the hands of a whole bunch of pundits out there.”

And he defended the trading unit, saying that “the C.I.O. has done a great job for a long extended period of time.”

The troubles are expected to weigh on the bank’s broader earnings. For example, the corporate group, which includes the chief investment office, is now expected to lose $800 million in the second quarter, the company said in the filing. Previously, JPMorgan estimated that the group would report net income of about $200 million.

Shares of JPMorgan tumbled 6.7 percent in after-hours trading. Its Wall Street rivals were also down sharply.

The $2 billion loss came from a complicated trading strategy that involved derivatives, financial instruments that derive their value from the prices of securities and other assets. JPMorgan said the derivatives trades were part of a hedge, meaning they were set up to offset potential losses on the bank’s large holdings of bonds and loans. But, in the sort of nightmare situation that bankers dread, the ostensible hedge backfired, producing losses of its own.

Since the financial crisis, several financial institutions have been rocked by risky trading, although under very different circumstances. At UBS. $2 billion in trading losses was attributed to a rogue trader, while MF Global collapsed not because of its bet on troubled European sovereign debt, but as a result of a loss of confidence that trade produced.

But JPMorgan’s trade was not so obviously fraught with risk. It produced large losses even without extreme movements in the derivatives markets or underlying bond markets. Indexes that track derivatives tied to corporate bonds have recently increased — reflecting a gloomier outlook for corporate bonds — but the move has not been jarring.

JPMorgan likely structured the trade in such a way that effectively magnified losses. Specifically, the bank bought insurance against losses on corporate debt through credit derivatives that increase in value if the underlying creditworthiness of companies is perceived to have deteriorated. But JPMorgan stumbled when it tried to modify that trade by also making an opposite bet with credit derivatives. Mr. Dimon said that the strategy “to reduce the credit hedge” was “poorly constructed and poorly monitored.”

The trades took place in the chief investment office of JPMorgan. These big trades are designed as hedges. Last month big derivatives trades executed in London by the unit appeared to be throwing markets out of kilter. The trades were reportedly put on by “the London whale” — a French trader, Bruno Iksil. On Thursday, Mr. Dimon said losses on the trade could grow.

The loss-making trades will further stoke the debate over whether banks make big bets under the guise of hedging. An old market adage says the best way to hedge is to simply sell an asset, rather than offset it with potentially dangerous derivatives. The Volcker Rule attempts to distinguish between true hedging and trading.

The losses will also raise enduring questions about JPMorgan’s ability to manage its risks. When it reported first-quarter earnings in April, it released a metric of risk-taking in the chief investment office that made it look as if the unit was taking risks in line with other parts of the bank. But on Thursday, the bank released a new measure of risk-taking that was nearly twice as high.

“JPMorgan Chase C.E.O. Jamie Dimon has been a relentless critic of financial reform,” said Dennis Kelleher, president of Better Markets, which supports tougher regulation of banks. The surprise loss, he said, “proves him wrong.”

Michael J. de la Merced contributed reporting.