Experts have cast doubt on HSBC's explanation that a software fault caused the computer glitch that left customers stranded for most of Monday.
Some eight million customers were unable to access their bank accounts between 11.30am on Monday and 1.00am on Tuesday.
An HSBC spokeswoman said that the firm had a problem with its database software that had prevented staff and customers from accessing customer details. Account holders trying to access details online were also left stranded.
She added that she could not confirm the supplier of the database software as "it would give information to criminals".
But Clive Longbottom, an analyst at Quocirca, told vnunet.com: "It's very strange that the system was down for so long. It is beyond the realm of general understanding that a database would take it upon itself to lock users out, and a standard fault in the software would corrupt at least some data."
"So, that points the finger at either an employee who has done something stupid or a very clever hacker who has shut out the bank's administrators," he continued. "I think it's very unlikely that the bank was hacked in this case. It's more likely to have been an error by an HSBC IT administrator."
Customers complained of being unable to determine whether salaries had been paid in on time, or whether cheques had been cashed.
HSBC stressed that no data had been lost and that the bank was "as confident as we can be" that the software issue had been resolved. "People could still draw cash out of the machines," said the spokeswoman.
Longbottom also thought it odd that ATMs continued to dispense cash. "I'm surprised they allowed customers to take out money if they couldn't check their balances," he said.
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