TSB has announced it plans to close 82 branches across the UK, affecting 370 jobs.
It comes as new chief executive Debbie Crosbie seeks to make £100m in savings by 2022.
It also said it would seek to help staff and customers affected by the decision.
The bank said it was making the closures because it had acquired a large network of branches from Lloyds Banking Group when it spun out, meaning it holds twice as many branches per 10,000 customers than competitors.
As customer behaviour continues to change, TSB said it has become difficult to sustain the number of branches it has.
Many have low customer numbers, it said, and transactions are decreasing year-on-year. In the last two years alone, TSB said it has seen branch transactions drop by 17 percent as customers increasingly choose to bank online or by phone.
Robin Bulloch, customer banking director at TSB, said: "We will fully support customers through this transition.
"We realise this is difficult news for our branch partners and will do everything to support those affected to offer voluntary redundancies and redeploy as many people as we can to other roles."