Ulster Bank says over 100,000 people are affected by a systems failure that's caused delayed automatic salary and social welfare payments to some of its accounts.
Earlier the bank said it expected the glitch to be resolved within 24 hours.
But now the banks says it cannot be sure how long fixing the problem will take, but it's assuring customers that all efforts are being made to resolve the issue quickly.
It has apologised to the affected customers and says it will ensure they do not suffer any financial loss as a result of the systems failure.
Una Dillon, head of card services at The Irish Payment Services Organisation, said: "We would consider this to be a very serious incident.
"Certainly in the whole history of the clearance system, this kind of technical problem has never happened before.
"This is the first of its kind. A huge amount of people have been affected. Other bank customers have also been affected because they would have expected salary payments."