Natwest's phone, mobile and internet banking systems have been downed by an IT problem and account balances have not updated overnight.
The system was due to undergo maintenance between 01:00am and 4:30am on Saturday morning, but a fault occurred and the systems are not accessible, Natwest press officer Michael Strachan told ZDNet UK on Saturday.
"We're sorry our digital, mobile and telephone banking services are currently unavailable," Natwest wrote on a page ZDNet UK saw after attempting to access online banking services. "We're working hard to get them back up and running as soon as possible. We would like to apologise for any inconvenience caused."
Natwest does not have a timetable for when the systems should be fixed. Cash machines and point-of-sale card readers are not affected, Strachan said. Mobile banking should be fixed by now, he said.
"This is not the result of an external hacking attack," he said. "There's just been a glitch between maintenance."
The glitch has caused an "offshoot" problem, Strachan said, which is that customer balances have not updated overnight.
"That won't affect many people, however for some people if you had no money in your account yesterday and there was due to be money credited overnight, that won't have happened... as far as the computer's concerned it's still last night."
"NatWest, I have a funny feeling our banking relationship will be over by the time you get you act together," NannyMcReed wrote in a tweet at 1:06pm on Saturday.
It could be "as much as a day before everything is up and running," Strachan told ZDNet UK on Saturday evening.