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.
Following the closures, 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 branches across the UK.
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 have soared by 99%.
The bank said that it will also spend £55m refurbishing 100 branches over the next two years, which will include a "focus on personal service, convenience and community engagement".
"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," commented Susan Allen, head of retail and business banking at Santander.
"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," Ms Allen said.
"We will support customers of closing branches to find alternative ways to bank with us that best suit their individual needs. We are also working alongside our unions to support colleagues through these changes and to find alternative roles for those impacted wherever possible," she added.
Ms Allen said the bank expects the size of its branch network to "remain stable for the foreseeable future."