After Friday's chaos following an HSBC banking systems outage, RBS had its own problems this weekend.
Customers complained they were unable to check accounts and in some cases that they were not credited as they should have been.
The bank said it had problems throughout Saturday due to major IT work that took place on Friday.
We are disappointed that we were not able to fully resolve all of the issues in the timescales that we wanted.
"We are disappointed that we were not able to fully resolve all of the issues in the timescales that we wanted," an RBS spokesperson said.
"Additional resources were mobilised, including extra staff in our branches on Saturday with a number of our branches extending their opening hours. We will keep our customers fully informed and apologise once again for the inconvenience this has caused."
Online banking and telephony are now running largely as normal, the spokesperson said. Account balances have also been updated.
Customers of Natwest, a member of the RBS group, also had issues, according to reports.
ATM, mobile and credit and debit card payment systems were operational in the case of RBS, but HSBC customers were not so lucky on Friday.
Customers flocked to Twitter to complain about the outage, with some shocked they could not use their cards at tills or at cash machines.
HSBC said the outage was caused by a server fault.
RBS has been doing some major IT work of late. It is currently in the process of virtualising thousands of its desktops.
