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Natwest's internet banking services appear to be completely broken, leaving people unable to check their account.
Customers also seem to be having issues making and seeing payments, with some people reporting that cash had left their account but wasn't making its way to the person they'd paid.
The problem has happened on the morning of most people's payday, and right before a bank holiday weekend.
Users report being unable to check their online banking either through Natwest's website or its app.
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And if customers attempt to ring the bank's phone services, they report being put into long queues for half an hour or more before they're able to speak to someone.
"Call centre down as well?" wrote one Twitter user. "Been on hold for 31 mins! My money has vanished into the abyss!"
The bank's official Twitter account posted to say that it was aware of problems with making payments. Its service status page also said Natwest is "aware that our Mobile Banking service is running slowly for some customers, we are working hard to resolve the issue and will keep you updated".
Natwest users complained that the app was prone to such problems.
"I've used a couple of bank apps — Natwest in the UK and Capital One in the States — and they seem to be quite glitchy and break a lot," said Ibrahim Salha, a Natwest customer and a colleague at The Independent. "I much prefer using Monzo, which is built with an app in mind, not as an afterthought. It's a lot quicker and actually has features you'd want to use, plus it looks like an app designed by humans, not an algorithm."
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