The problems also affected customers of the Royal Bank of Scotland (RBS), which is in the same banking group.
NatWest, whose slogan is Helpful Banking, has already been forced to apologise twice over the last couple of months for problems with the app, with the most recent glitch happening only a week ago.
One Twitter user wrote "Oh yeah, it's Friday again! App broken" while another complained that problems with the app seemed to be a "weekly occurrence".
An RBS and NatWest spokeswoman said on Friday: "We've had huge demand for our app, which led to some customers experiencing the app running slowly for around half an hour. We apologise for the inconvenience caused."
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