Taxpayer-backed Royal Bank of Scotland is closing 44 branches across the UK - including 14 classed as "last banks in town".
The bailed-out lender said the move comes after a 30% drop in branch transactions since 2010.
RBS, which is just over 80% owned by the Government, said the so-called last banks in town that are closing are generally only open for a few hours a week and see one or two customers an hour.
But campaigners said the group was letting customers down.
A spokesman for RBS said: "Banking has changed significantly over the last few years as more and more of our customers are banking with us where and when it is convenient for them."
He added: "We have to adapt to what our customers want, which is why we're investing in a range of other ways our customers can bank with us, including online and telephone banking, our mobile app, and in any one of the Post Office's 11,500 branches across the UK."
RBS said it had been telling affected customers about the closures over the past two months.
Campaign group Move Your Money said RBS had "consistently undermined the interests of its customers and wider society since being bailed out in 2008".
"Banks of this scale just can't be trusted to take its customers' needs into account, even when the only reason it's still around is because of our support."
The branch closures come after RBS said in February it planned to slash costs by more than £5 billion over the next three to four years after slumping into the red by £8.2 billion in 2013.
It has around 2,000 branches across the UK, although it is spinning 314 of those off into its Williams & Glyn brand to meet European Union rules on state aid.
The group announced in February that customers of RBS as well as its sister banks NatWest and Ulster Bank would be able to use any of the Post Office's 11,500 branches to pay cheques or cash into their accounts from later this year.
RBS already has a relationship with the Post Office which enables its customers to withdraw cash and check their balances at Post Office branches.