Natwest's online and mobile banking services have gone down, the bank has said.
NatWest is owned by Royal Bank of Scotland, and some customers of RBS are also reporting issues with the digital and mobile banking services. Ulster Bank services are similarly affected.
Meanwhile, the RBS website has crashed.
An RBS spokesperson said: “We are aware that customers are currently experiencing issue logging into their online and mobile banking accounts.
"We would like to apologise for the inconvenience. We are aware of the problem and are working to fix the issue. Customers can still use ATMs and telephone banking.”
RBS chief executive Ross McEwan told LBC radio that the bank did not know what was causing the problems.
"We're still working through what the issue is,” he said. "We make a lot of changes to our technology on an ongoing basis, it may be related to that, but we just don't know at this point."
An RBS spokesperson said: “We are aware that customers are currently experiencing issue logging into their online and mobile banking accounts.
"We would like to apologise for the inconvenience. We are aware of the problem and are working to fix the issue. Customers can still use ATMs and telephone banking.”
In a statement sent to customers, Natwest apologised after users were left locked out of their accounts.
"We are sorry if you are having trouble getting into our online banking or mobile app," it said.
"We are working hard to resolve the issue. You can still access your account through telephone banking or alternatively you can visit one of our ATMs."
Some customers have also complained that their NatWest cards are not working at ATMs, leaving them with no access to their cash.
The glitch came a day after some Barclays customers were left struggling to log in to their accounts due to a technical problem.
RBS has recently announced hundreds of branch closures. In May, the lender said it would shut 162 branches, leading to 792 job cuts, and earlier this month the bank announced a further 54 closures, with an additional 258 job losses.
