Ulster Bank lost 2,587 customers in the final quarter of 2023, new data from the Current Account Switch Service (Cass) shows.
The account changes came during a period late last year, when Ulster Bank announced plans to close ten branches across Northern Ireland.
Nine of the branches have already closed, with the tenth in Downpatrick, due to go in November.
The Cass data shows Bank of Ireland lost 563 via its switch service between October 1 and December 31 2023, while the AIB Group lost 807.
Danske Bank was down by 236 customers over the period.
UK-wide, Nationwide Building Society, Barclays and Lloyds Bank made the biggest gains through customers using the service.
Nationwide made 163,363 net gains in terms of full account switches. Barclays made 12,823 net gains, while Lloyds made 5,800 in the three month period.
At the other end of the spectrum, NatWest made 43,182 net switching losses, RBS lost 11,621, Halifax lost 41,144 and Santander lost 34,581.
The figures do not include current account switches made outside the service, which ensures that payments are automatically moved to the new bank account and offers a guarantee that customers will not be left out of pocket if anything goes wrong.
Andrew Hagger, a personal finance expert and founder of Moneycomms.co.uk, said: ”Nationwide blew the competition away” in the fourth quarter of last year.
He suggested that a £200 switching incentive from Nationwide last autumn may have helped attract customers.
Mr Hagger added: “Consumers are happy to jump ship for the next freebie - particularly as people are still feeling the squeeze from higher bills.”
Here are the net switching gains or losses made in the fourth quarter of 2023 by banks, according to figures published by Cass:
- AIB Group (includes Allied Irish Bank brand switches), minus 807
- Bank Of Ireland, minus 563
- Bank of Scotland, minus 3,024
- Barclays, 12,823
- Co-operative Bank (includes Smile brand switches), minus 4,238
- Danske, minus 236
- Halifax, minus 41,144
- HSBC (includes First Direct brand switches), minus 3,263
- JP Morgan Chase, minus 3,055
- Lloyds Bank 5,800
- Monzo, minus 5,336
- Nationwide, 163,363
- NatWest, minus 43,182
- RBS (includes Coutts and Isle of Man brand switches), minus 11,621
- Santander, minus 34,581
- Starling Bank, minus 1,712
- Triodos Bank, minus 25
- TSB, minus 10,917
- Ulster Bank, minus 2,587
- Virgin Money, minus 8,592