Ulster Bank boss Jim Brown has admitted that up to 600,000 of its customers in Ireland have been affected by the ban's recent and on-going IT debacle.
The bank had previously estimated that around only 100,000 account holders were impacted on by the June 19 technical difficulties.
Mr Brown, who is appearing before the Oireachtas Finance Committee, said that around half of the bank's customers in the south were hit by the glitch.
"We won't know the exact number (of customers affected) until we get through processing all the transactions," he said, "but I would guesstimate that it would be (around) half of our customer base, which in the Republic is probably be 500,000 to 600,000 but we don't know the exact number."
Meanwhile, Mr Brown said the European Central Bank's .25% interest rate cut would be passed on to Ulster Bank's tracker and variable mortgage customers.
he also refused to rule out accepting a bonus for 2012, and said he would make a "judgement call" at the end of the year.