ULSTER Bank services are back up and running, it said today.
Last night the bank was forced to apologise after customers were left unable to use their credit or debit cards, or access their online accounts yesterday evening.
It is undesrtood that thousands were affected by the problem.
ATMs were inaccessible and credit cards were rejected on what was one of the busiest shopping days of the year.
The problem came just over a year since Ulster Bank customers were locked out of their online accounts for up to a month during a catastrophic IT breakdown, which resulted in the bank having to compensate many customers.
Customers took to Twitter last night to vent their frustration and fears that a system-wide computer glitch that left 12 million customers in the UK and Ireland unable to access their money in the summer of 2012 was being repeated.
"We are very sorry for the system issues that affected our customers this evening," said a spokeswoman for the bank last night.
"Our customers are reporting that services are coming back on line. We will confirm when all systems have returned to normal service.
"If customers have been left out of pocket as a result of these system problems, we will put this right. If any customer is unable to resolve an issue caused by the disruption, they should get in touch with our call centres or come into a branch in the morning where our staff will be ready to help."
However she could not clarify how many customers were affected or what caused the problem.
Last night many customers reacted with fury, both here and in the UK, especially on what was dubbed "Cyber Monday" when online retailers expected one of the busiest days of the Christmas season.
Customers spoke of being left stranded at tills with no means of paying or having to go without food or other necessities. Others were furious over the embarrassment caused when they were unable to pay their bills at restaurants or petrol stations.
"Groundhog day at RBS. Angry bank customers. Again. Natwest and Ulster Bank systems also down. Again," tweeted the UK website Move Your Money UK.
"Ulster Bank gone haywire again, online and ATMs apparently," said another Irish customer.
The problem appeared to have affected only Ulster bank debit and credit cards as customers of other banks were still able to withdraw cash from Ulster Bank ATMs using their own bank cards.
RBS said that they were trying to fix the problem "urgently." It comes after faults were recorded on Friday where customers couldn't access their online banking. Initial reports online pointed towards server issues with the parent group bank.The IT meltdown of 2012 saw customers locked out of their accounts for up to a month. The official incident period of 2012 was from June 19 to July 18 but it took months for the various payments to be realigned. A total of 750,000 customers were affected during the catastrophic IT failure and people who incurred fees for late payments of loans and mortgages were compensated.