RBS Group said customers of its brands NatWest, Royal Bank of Scotland and Ulster Bank struggled to log into its mobile banking app, with the problems lasting for around six hours.
In an unconnected incident, Lloyds Banking Group said a "small number" of Lloyds, Halifax and Bank of Scotland customers were affected by an IT issue which meant they had trouble accessing payments.
The Lloyds issue related to telephone and internet banking only and lasted for around an hour and-a-half this morning.
RBS said in a statement: "Some customers may have had trouble getting into mobile banking today between 08.40-14.30. All services are back up and running as normal. We apologise for the inconvenience caused."
A spokesman for Lloyds said: "We are aware that a small number of customers experienced issues accessing payments this morning. The issue has now been rectified and we are working with those customers who were affected."
Customers vented their annoyance on Twitter, and pointed out the issues had arisen as people gear up for the Bank Holiday weekend.
One complained of NatWest: "Pay day Bank Holiday no mobile service."