A technical problem that left hundreds of thousands of Ulster Bank customers without access to their money has been resolved, the bank has said.
It apologised for Monday evening's problems and said it would compensate anyone left out of pocket.
Customers said they could not use cash machines, debit cards or access online or telephone banking.
Up to 750,000 customers from RBS, Natwest and Ulster Bank - the Royal Bank of Scotland group - were affected.
All systems are now back to normal, the bank has said.
The group said it was "very sorry for the system issues" but, as yet, it is unclear exactly what the "glitch" was.
In a statement issued on Tuesday, the bank said: "The systems issues that affected our customers last night have now been resolved and all our services are now back working normally.
"We would like to apologise to our customers. If anyone has been left out of pocket as a result of these systems problems, we will put this right.
"If any customer is experiencing issues this morning, they should get in touch with our call centres or come into our branches, where our staff will be ready to help."
A group spokesperson added: "If anyone has been left out of pocket as a result of these systems problems, we will put this right."
Susan Allen, RBS director of customer relations, said: "We are deeply sorry for the inconvenience."
Ms Allen told BBC Radio 4's Today programme that the group still did not know the cause of the glitch, but was working through "a detailed analysis".
"It is completely unacceptable that customers couldn't access their own money," she added.
In 2012, an IT fault locked huge numbers of RBS customers out of their accounts.
In Northern Ireland, thousands were unable to withdraw cash or access their Ulster Bank accounts for weeks after a software upgrade on 19 June.
Ulster Bank paid out more than £18m in compensation to 300,000 people affected in Northern Ireland by that glitch.
The RBS group, which is 80%-owned by the UK government, said it had improved its services since the problems occurred in June and July of 2012.
Monday 2 December has been dubbed Cyber Monday. It is considered one of the busiest online shopping days of the year due to the flurry of discounts offered by retailers hoping to boost their festive sales.