Customers reported problems using services since early this morning, until issues were solved around 10.30am.
It comes just a day after competitor Barclays had IT problems that locked customers out of their accounts for several hours.
A spokesman for NatWest and RBS said: “We would like to apologise to customers who experienced issues logging into their online and mobile banking accounts this morning. The issue has now been resolved and no customer will be left out of pocket as result of this.”
Angry customers had taken to social media to complain about the outage on a day that many are due to be paid salaries – some also reported problems using their cards.
One user Chrissie wrote on Twitter: “NatWest closed my local branch. Can’t use the app to see how much money I have for petrol to travel 15 miles to my nearest branch. They made everyone go online. We have every right to moan.”
Another Julian Hammond wrote: “We run a small family business and are due to pay the staff today, please advise how we can do this, if we cannot access the online banking services. Thank you.”
Earlier this month TSB boss Paul Pester stepped down from the bank after IT issues continued to cripple its services.