As many as 130 jobs will be lost as First Trust Bank closes half of its branches in Northern Ireland in 2017.
On Wednesday, it said it was starting a "reshaping and investment programme" involving the "consolidation of First Trust Bank's branch network" with the closure of 15 of its 30 branches.
The Financial Service Union (FSU) said up to 130 jobs are at risk.
The union's Larry Broderick said he had received an assurance there would be no compulsory redundancies.
First Trust, which is owned by Dublin-based AIB, is one of the so-called "big four" banks in Northern Ireland.
The head of First Trust, Des Moore, said the closures had been a difficult decision.
Analysis: BBC News NI Economics and Business Editor John Campbell
Just five years ago First Trust had more than 40 branches - when this latest cull is complete it will have just 15.
Closing branches is part of a trend across the banking industry as customers do more of their transactions online.
But it is also worth looking at the particular situation at First Trust and its parent company AIB.
AIB is owned by the Irish state and it's planning to sell 25% of the bank's shares in the near future.
The bank insists the branch closures aren't related - but stripping out costs ahead of the sale will do no harm to the share price.
"Since 2010 we have seen almost a 40% decline in branch usage coupled with a 117% increase in mobile transactions over the past two years and we have had to make the difficult decision to close a number of branches," he said.
"We will be working hard to minimise the impact on the customers and staff concerned."
First Trust said the closures were designed to ensure a sustainable future for the bank and its customers, addressing the considerable shift in customer behaviour and their changing needs.
It also announced a £10m investment strategy for personal and business customers, including five new business centres across Northern Ireland.
First Trust branch closure dates
- Antrim - 7 July
- Andersonstown - 7 July
- Armagh - 7 July
- Banbridge - 4 August
- Cookstown - 30 June
- Crescent Link, Derry - 7 July
- Downpatrick - 28 July
- High Street, Belfast - 4 August
- Knock, Upper Newtownards Road - 28 July
- Magherafelt - 28 July
- Newtownards - 30 June
- Ormeau Road, Belfast - 30 June
- Portadown - 4 August
- Strabane - 28 July
- Warrenpoint - 30 June
The bank has also agreed a partnership with the Post Office enabling customers to bank in any of the 500 Post Offices in Northern Ireland.
The FSU's Mr Broderick described the scale of First Trust Bank branch closures as "a serious blow for staff, customers and the Northern Ireland economy".
According to Mr Broderick, "while staff were aware of an ongoing restructuring plan at FTB the scale of closures announced today has come as a shock".
"Great teams in bank branches, many who have served their community for many years, now face the prospect of closure and relocation. It is a major blow."