Staff at some Santander branches were unable to access some customer accounts owing to software problems.
The bank said that 38 of the 1,400 branches across the country were affected when a software changeover caused the computer difficulties.
The bank apologised for the hitch, which affected customers in branches between 1000 BST and 1030 BST, and said the problem had now been resolved.
Internet banking and cash machines were unaffected.
Although banks suffer relatively regular server problems which can affect their online banking operations, a UK-wide issue with computers in a branch network is unusual.
Staff were unable to access certain account information for customers who visited branches.
"For those who did experience a problem, we are sorry for the inconvenience," a Santander spokesman said.
He added that although "behind-the-scenes work" had caused the bank's system to slow down for about 15 minutes, there had been no actual shutdown and only "a small number" of branches had been affected.
The spokesman said this was a routine upgrade of computer software.
The transfer of records from Alliance and Leicester, Bradford and Bingley, and Abbey customers - which required a major overhaul of Santander's computer system - was completed some time ago.
The bank has suffered from mechanical problems before.
In December, Santander admitted that up to 35,000 people could have received other customers' details on bank statements.
A printing error led to pages from other customers' statements appearing on paperwork sent to current account holders. As a result, recipients were sent details of their own transactions and other people's as well.
In a separate case, Southern Railway has apologised to customers unable to access its online booking system.
It blamed unprecedented demand at the start of a sale, which has now been extended to four days, of advanced rail tickets.