Yesterday, customers were locked out of their online, telephone and branch accounts for several hours, apparently due to a computer glitch.
"We are aware that customers are now experiencing issues logging into their online and mobile banking accounts".
Ulster Bank has said it is having technical issues and has advised customers that they may be unable to access some services.
RBS confirmed that customers across its three consumer brands were affected by the outage.
"As you may be aware we've made a lot of changes to our technology on an ongoing basis, it may be related to that".
Britain's second largest high street bank has been forced to apologise to the four million people who access their accounts via the website or app.
Hannah Maundrell, editor of money.co.uk said: "Banks really need to pull their socks up because this keeps happening".
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"We are aware that customers are experiencing issues and are working to fix it".
Customers of RBS, NatWest and Ulster Bank are now unable to access their accounts through the banking group's online and app platforms. "You can still access your account through telephone banking or alternatively you can visit one of our ATMs".
The outages come when Britain's lenders have been driving customers towards such services as the banks cull branch networks to cut costs.
'We would like to apologise for the inconvenience. Millions across the United Kingdom and Ireland were said to be affected as the issued also affected Nat West and RBS customers.
The latest banking problems follow TSB's huge IT meltdown earlier this year, after a botched IT switch locked millions of customers out of accounts. Not really acceptable for our 24 economy!
The consequences for banks of online banking problems and the subsequent scrutiny of those failings by politicians can be severe. How can we trust you with our money?'