Royal Bank of Scotland says it has fixed its latest IT failure, which affected two million of the group's customers ahead of the Easter weekend.
Users of the NatWest, RBS and Ulster Bank mobile app found they were locked out of the service this morning, prompting the bank to issue an apology as it worked to resolve the problem.
Five hours later the bank released a statement which said: "All our mobile banking applications are now running normally. We apologise again for the inconvenience caused to our customers."
There was no reason given for the outage.
There are more than two million active users of the app among RBS and NatWest personal banking customers alone, with 13 million log-ins taking place each week.
App users have taken to Twitter to vent their frustration Customers vented their anger at being let down again by the group's technology with one Twitter user writing: "No access to my iPhone app think it's time to change banks!"
The latest IT issues came to light just three weeks after a hardware fault prevented RBS Group customers from using cash machines and also affected online and telephone banking services.
The group had earlier taken a £175m hit as a result of IT chaos last summer which left NatWest, RBS and Ulster Bank customers locked out of their accounts. The bulk of the redress costs related to Ulster Bank, where the problems took weeks longer to clear up.
Last October, NatWest had to suspend a feature on its mobile phone app called GetCash, after the service was subject to a spate of so-called "phishing" attacks by fraudsters.
The GetCash service, which allows customers to withdraw cash without using a debit card, was later re-instated after security was bolstered.
Earlier this month the bank announced a new mobile banking feature for RBS/NatWest customers called Pay Your Contacts, which allows them to send payments of up to £100 to anyone with a Visa card, by entering their mobile number.
A spokeswoman for consumer group Which? said all the problems would raise wider questions about how robust and up to date banks' IT systems were.
She said: "Consumers and businesses alike rely on mobile banking services to access their accounts and consumers will rightly want to be assured that their money is accessible and safe at all times."
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