TSB chief executive Paul Pester apologised to customers on Tuesday after an upgrade to its online banking system resulted in a data breach.
The bank took its online banking systems offline around 10.30am and hopes to be back up later this afternoon, Pester said.
“I’ve just resurfaced after 48 hours with my teams who have been working as hard and fast as they can to get our services back up and running,” he said.
“This isn’t the level of service that we pride ourselves on providing, and isn’t what our customers have come to expect from TSB, and for that I’m truly sorry.”
The data breach allowed some customers to see other customers' accounts while others reported being unable to access their accounts and make payments.
What caused the disruption
The problems began on Saturday when the lender upgraded its internet banking platform.
The lender, which is now owned by Spain’s Banco Sabadell, moved its customers’ data over to the new platform over the weekend.
Customers were told online banking and payment systems would be unavailable between 4pm Friday and 6pm Sunday but many customers were still unable to access their accounts on Tuesday.
"We’re still seeing issues with access to our digital services,” Pester said. “One of the steps we need to take to resolve this is to take our mobile app and online banking down for a few hours.”
MP demands answers
Pester’s remarks come after he received a letter from Nicky Morgan, the chair of the Treasury Select Committee, to ask what went wrong, the extent of the failure and how the bank intends to compensate affected customers.
“It simply isn't good enough to expose customers to IT failures, including delays in paying bills and an inability to access their own money,” Morgan said.
"Warm words and platitudes will not suffice. TSB customers deserve to know what has happened, when normal services will resume, and how they can expect to be compensated."
One TSB customer said he was given access to someone else’s £35,000 savings account and £11,000 ISA when he logged online to check how much he had spent at the pub the night before.
Another customer said he could see other people's accounts, totalling more than £20,000.