The "data management" problem occurred earlier this week and took over 12 hours to fix. It prevented customers from accessing their account information over the phone or Web.
Customers of HSBC's telephone banking service First Direct were also hit by the failure.
A spokeswoman for HSBC said the database system at fault was developed in-house as a bespoke system, with support from IBM. She stressed that customers could still access account information at HSBC branches and withdraw money from ATMs. She added that security had not been breached by the IT failure. IBMrefused to comment.
HSBC's IT problem sees the bank join a growing number of high street banks to have suffered technology failures. Online services from Egg and Barclays have both suffered security glitches, and the Web site of Internet bank Cahoot crashed on the day of its launch.