RBS had been tweeting customers directly on Twitter saying it is aware of some issues and that it was working to fix them.
NatWest, owned by RBS, also experienced the same issues.
We’re aware of some issues on our Mobile Banking service and are working hard to fix them. Sorry and thanks for your patience.— NatWest (@NatWest_Help) December 20, 2016
The timing of the glitch comes at a crucial time in people's personal finances in the run up to Christmas.
It is not yet clear how many people have been affected and the both banks have apologised.
Sorry if you had trouble getting into Mobile Banking earlier today. Our service is now running as normal, thanks for your patience.— NatWest (@NatWest_Help) December 20, 2016
RBS has been contacted for comment.