Santander is to close 140 branches in the UK, putting more than 1,200 jobs at risk, following a slump in the number of transactions it handles.
The bank said it is consulting with unions and will seek to find alternative roles for 1,270 employees affected by the closures. It expects to redeploy about a third across the group.
The branches involved includes those that are not visited frequently and are in close proximity to another branch.
Following the closures by the end of the year, Santander will retain a network of 614 branches with its customers also having the option to bank using the more than 11,000 Post Office sites across the UK.
Reduction in high street banks
The move comes as the number of transactions carried out at Santander branches fell by 23% over the past three years, while transactions online and on mobile phones soared by 99%.
According to consumer group Which?, nearly two thirds of the UK's bank branch network has been lost over the past 30 years.
Banks and building societies had 20,583 branches in 1988 according to parliamentary records, but the consumer group's analysis of current account providers suggests the figure has dwindled to 7,586 branches.
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Digital-only banks like Monzo and Starling also present a challenge for traditional high-street banks.
Susan Allen, head of retail and business banking at Santander, said: "The way our customers are choosing to bank with us has changed dramatically in recent years, with more and more customers using online and mobile channels.
"As a result, we have had to take some very difficult decisions over our less visited branches, and those where we have other branches in close proximity."
She added that the bank expects the size of its branch network to "remain stable for the foreseeable future."
The lender, which is owned by Spanish banking giant Banco Santander, has around 15 million customers and as at the end of last June employed around 24,200 people.