RBS and NatWest suffered major problems with their websites as the UK went back to work after the Bank Holiday weekend.
The sites stopped working at around 09:00 BST according to reports and were still out of action five hours later.
Both banks tweeted that customers could use other methods to access accounts.
But some people were complaining on Twitter that they were still unable to complete transactions, with one tweeting NatWest to say: “It’s like the money is disappearing.”
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