Barclays cash machines, online banking and some counter services were inaccessible after a hardware failure today.
Cash machines in the south of the country were not working and no services were available through telephone and online banking. Some services were unavailable at branch counters.
A spokesperson for Barclays said the problems had been caused by a failed disc array - a collection of hard drives built for storage. Engineers are currently working on getting "replacement cards", the spokesperson said.
Barclays added, "There is a technical issue affecting some of our systems. We have got engineers on site right now replacing the hardware."
The hardware problems started at around 1pm on Tuesday, causing several systems to suffer. Customers in different parts of the country are affected in different ways, but telephone and online banking systems were down across the UK.
"Customers will be able to speak to online banking staff, but no payments can be processed. It is possible to take money out over branch counters," a Barclays spokesperson said.
The bank said that it is hoping to fix the problems by the end of the day. A message on its personal banking site read, "We're currently experiencing technical problems in Online Banking that means some of you may not be able to log in to the service. We apologise for any inconvenience this may cause and suggest you try logging in again later."