The Royal Bank of Scotland and NatWest banking app failed to work for nearly an hour on Friday morning.
The RBS Group said in a statement that both app and online banking services were still running slowly.
Many customers have been complaining on social media that payments and money transfers have not been getting through.
An RBS spokesman said that some of its customers were affected, but not the majority.
He added that no customer would be left out of pocket.
The bank has not yet issued an explanation for the problem.