RBS customers were left unable to login to their online banking this morning.
Frustrated customers used Twitter to complain of problems accessing their account online and making payments.
The issue, which lasted around 50 minutes and affected RBS, NatWest and Ulster Bank customers, has since been rectified.
An RBS spokesperson said: "We are aware that some customers experienced issues with online banking this morning. This has now been resolved and our service is operating as normal. We apologise to those customers who experienced difficulties."
The bank said it had yet to determine the cause.
It marks the latest in a series of IT issues that have beset RBS. Just last month, 600,000 customers were left fuming after payments either never arrived or failed to send.
Last year the bank was fined £56 million by regulators for an issue dating back to 2012, when customers were left unable to access their accounts.