HSBC has closed more than a quarter of its UK branches over the last two years, as part of 1,000 bank closures in total, research by Which? has found.
The consumer organisation says that HSBC closed 321 branches since January 2015, or 27% of its network.
The much smaller Co-operative Bank has closed more than half of its branches over the same period.
HSBC said most customers now bank via the internet or on smartphones.
However, Which? said that while 56% of adults used online banking last year, there are still 20 million adults who don't - or can't - use it.
Many of those have a poor broadband connection.
Indeed, the areas most affected by closures - South West England, Wales and Scotland - are predominantly rural and often with unreliable connectivity.
Lloyds has already announced that it plans to speed up its closure programme in 2017, closing about 200 branches.
HSBC said it would continue to review its branch network, to make sure it has a sustainable network for the future.
But it said far fewer people were going into branches.
"Overall footfall in our branches has fallen by over 40%, with 93% of contact with the bank now completed by telephone, internet or smartphone, plus 97% of cash withdrawals are made via an ATM," said an HSBC spokesperson.
"When we do make the decision to close a branch, our main priority is to ensure that our customers and their banking needs are catered for in the best way possible."
The British Bankers Association (BBA) is in the process of updating guidelines for branch closures.
Those guidelines will ensure that staff in affected banks are properly trained, and that communities are given fair warning when a branch is closing.
But Which? said banks could still do more to help customers faced with closures.
"Banks can and must do a better job of working with their customers to understand their needs and those of the local community, especially when they are making changes to the services they offer or closing branches," said Peter Vicary-Smith, the chief executive of Which?
The figures used by Which? came from the banks themselves.
Research by the BBC found that High Street banks closed 600 branches in the year to May 2015.