Millions of customers who bank online with JPMorgan Chase & Co. lost electronic access to their accounts for more than 22 hours as the company's website suffered an outage Monday and Tuesday.

A spokesman for the nation's second-largest bank by assets said the service went down because of a "technical issue, not hackers or criminals." Another person familiar with the situation said the disruption was the result of a flaw in a software program tailored for JPMorgan.

It took the bank much of Tuesday to discover the flaw and figure out how to fix it across the system, this person said. The bank expected the online service, which affected computer and iPhone users, to be fully restored Tuesday evening, this person added.

But that wasn't soon enough for David Daniels, who was unable Monday night to access the checking- and savings-account balances he set up for his Salt Lake City-based photography business. Almost 24 hours later, he was still out of luck and holding off on buying a camera.

"I can't buy a camera if I can't see how much money I have," said Mr. Daniels, who checks his balances at least once a day.

JPMorgan Chase Chief Executive James Dimon, who on Tuesday promoted a new application for Apple Inc.'s iPhone that allows customers to deposit checks over the phone, apologized for the system shutdown while speaking at an investors conference.

More than 16 million customers in the bank's Chase retail unit use the online system to pay bills or view account information. A spokesman said the bank "will work with customers" who were unable to make payments on time because of the shutdown.

Despite concerns over security, online banking has blossomed over the past decade as the convenience of electronic bill payment has won converts. More people in the U.S. are relying on the Web for their banking as financial institutions prod customers to use the cheaper service outlet as an alternative to a local branch. Some 51% of consumers now prefer online banking to other methods, up from 44% in 2008, according to a study issued last month by Mercator Advisory Group, a consulting firm that specializes in the financial-services industry.

While websites suffer outages all the time, the duration of Chase's disruption was longer than many. Patricia McGinnis, research director in Mercator's banking group, said the Chase outage appeared to be unusually lengthy.

"It's not uncommon for there to be problems when you update an operating system or piece of software," she said. "Sometimes there is a little glitch when something gets changed, but there should be backup available, particularly when something is used so heavily by so many customers."

Lane Hartwell, a photographer in Alameda, Calif., is used to checking her Chase bank balance on her iPhone daily, so she was surprised yesterday afternoon to find she couldn't access her checking-account information. Ms. Hartwell was waiting for a check to clear before she purchased photography equipment she needed for a shoot this weekend. She delayed making the purchase and checked again Monday around 8 p.m.

"I was so frustrated because I couldn't order the equipment since I had no idea what the balance was," she says.

Ms. Hartwell called her branch and was able to find out her account balance on Tuesday, but she's now stuck with higher shipping costs.

"I had to order it today and have it rushed, which is more money out of my pocket because of Chase's problems," she said.

Meanwhile, at the same conference where Mr. Dimon apologized for the shutdown, he said lending costs for bank customers will go up after implementing stricter capital and liquidity requirements from the Basel III capital accord.

Mr. Dimon, speaking at Barclays Capital's financial-services conference in New York, said his bank "will be fine" even after implementing stricter capital and liquidity requirements from the Basel Committee on Banking Supervision.

The CEO also said the capital-markets trading business so far this quarter "has been stable for us" and similar to the second quarter's. He said third-quarter mortgage losses are likely flat from second-quarter losses. Overall, "credit is in very, very good shape," he said.

He reiterated that the Volcker rule in the Dodd-Frank act won't limit JPMorgan's ability to do business with customers. But the bank will be moving its commodities and credit-default-swap businesses to a separate subsidiary, which he called an organizational "nightmare."

JPMorgan has already given notice to its proprietary traders that their business will be wound down.