Britain's TSB bank, owned by Spain's Banco Sabadell, took down its mobile app and online services on Tuesday in an attempt to fix technical problems which have locked customers out of their accounts.
The bank said about 402 customers on Sunday were able to see data that they would not usually see online, often of so called "connected accounts" that family members typically use.
TSB Chief Executive Officer Paul Pester said he had worked for 48 hours to fix the problem but that the bank had to take down both the mobile app and online banking again on Tuesday. The services should be working later today, he said.
"I'm truly sorry," Pester said in a statement. "Customers can rest assured that no one will be left out of pocket as a result of these service issues."
Pester has written to each customer to apologize.