America's largest bank JPMorgan Chase lost two billion dollars (£1.24bn) in the past six weeks in a trading portfolio designed to hedge against risks the company takes with its own money.
The company's stock plunged almost 7% in after-hours trading after the loss was announced. Other bank stocks, including Citigroup and Bank of America, suffered heavy losses as well.
"The portfolio has proved to be riskier, more volatile and less effective as an economic hedge than we thought," chief executive Jamie Dimon said. "There were many errors, sloppiness and bad judgment."
The trading loss is an embarrassment for a bank that came through the 2008 financial crisis in much better health than its peers, keeping clear of risky investments that hurt many other banks.
The loss came in a portfolio of the complex financial instruments known as derivatives, and in a division of JPMorgan designed to help control its exposure to risk in the financial markets and invest excess money in its corporate treasury.
Bloomberg News reported in April that a single JPMorgan trader in London, known in the bond market as "the London whale", was making such large trades that he was moving prices in the 10 trillion-dollar market. Mr Dimon said the losses were "somewhat related" to that story, but seemed to suggest that the problem was broader. Mr Dimon also said the company had "acted too defensively" and should have looked into the division more closely.
The Wall Street Journal reported last month that JPMorgan had invested heavily in an index of credit-default swaps, insurance-like products that protect against default by bond issuers.
Hedge funds were betting that the index would lose value, forcing JPMorgan to sell investments at a loss. The losses came in part because financial markets have been far more volatile since the end of March.
Partly because of the two billion trading loss, JPMorgan said it expected a loss of 800 million dollars (£497 million) this quarter for a segment of its business known as corporate and private equity. It had planned on a profit for the segment of 200 million dollars (£124 million).
The loss is expected to hurt JPMorgan's overall earnings for the second quarter, which ends on June 30. Mr Dimon apologised for the losses, which he said occurred since the first quarter, which ended March 31.