Hundreds more which remain open have slashed their opening hours, with some just open for one or two days a week.
There were 3,303 bank branch closures, equating to 34 per cent of the network, between January 2015 and August 2019, the consumer group said.
The network has reduced in size from 9,803 branches to 6,549 over the period, with the number of closures being slightly offset by 49 branches also opening.
Of the UK's bank branches that remain open, 298 are now operating with reduced opening hours of four days a week or less, Which? said.
Eleven of these branches - all in Scotland - open for just one day a week, while 45 branches nationwide open for just two days per week.
The shrinking branch network has been driven by a wave of closures by major high street names.
Which? said NatWest - part of the RBS Group - closed 638 branches, while sister bank RBS has shut 412 - equating to 74 per cent of its UK branches.
HSBC made 442 closures and Barclays shut at least 481 branches - Which? said Barclays did not share full bank closure numbers with it, so the total could be higher.
Within Lloyds Banking Group, Lloyds Bank closed 404 branches, while Bank of Scotland shut 95 branches and Halifax shed 70 of its branches.
Meanwhile, Santander closed 294 branches, the Co-op lost 152 and Yorkshire Bank shut 74, Which? said.
Which? said that by contrast, Nationwide Building Society has retained an “impressive” 96 per cent of its branches.
Nationwide has pledged not to leave any town or city in which it is currently based without a branch until at least May 2021.
Rural areas have been particularly hard hit by the branch closures, at a time when ATM shutdowns are also fuelling concerns about free access to cash.
For example, the consumer group could not find any bank branches in the entire Wentworth and Dearne constituency in Yorkshire, which has a population of 98,000.
North East Derbyshire and Stoke-on-Trent North have both lost six branches, equating to more than four-fifths of their network, Which? said.
Central Devon saw an 81 per cent branch reduction, while Carmarthen East and Dinefwr lost four-fifths (80 per cent) of the network.
Wales has seen 43 per cent of its bank branches shut, while for Scotland the percentage is 38 per cent, and in Northern Ireland it is 29 per cent, Which? said.
It argued that while some banks offer mobile branches, they do not offer the same convenience of access as a permanent bank branch.
Which? recently wrote to Chancellor Sajid Javid calling for action to guarantee people's ability to access and pay with cash.
Jenny Ross, Which? money editor, said: “Banks are closing their branches at an alarming rate, which risks shutting many people out of vital financial services and affecting their ability to access their own cash.
“Bank branches play a crucial role within communities, serving consumers and businesses alike. The industry must ensure no-one is left behind by the digital transition and that when banks shut their doors, they don't shut their customers out of important banking services.”
Which? said it used several sources to record the data.
Martin Kearsley, banking director at the Post Office, said: “Almost every bank customer - whether personal or small business - can access their usual high street bank account to get cash out at any Post Office branch, make a cash or cheque deposit or check their balance.
“The services we offer are vital, especially for rural and remote communities, and they provide an opportunity for face-to-face interaction, which is important to many.”
Here are the numbers of bank branches closed between January 2015 and August 2019, according to Which? Figures show the number of branch closures followed by the number opened and the percentage of branches closed between January 2015 and August 2019 (Which? said Barclays did not give full data):
- RBS, 412, 0, 74%
- Co-op, 152, 0, 69%
- First Trust (Northern Ireland), 15, 0, 50%
- NatWest, 638, 0, 49%
- Clydesdale Bank, 60, 0, 47%
- Yorkshire Bank, 74, 0, 45%
- Ulster Bank (Northern Ireland), 35, 0, 44%
- HSBC, 442, 1, 42%
- Barclays, 481, 4, 33%
- Bank of Scotland, 95, 0, 32%
- Santander, 294, 1, 32%
- Lloyds, 404, 0, 31%
- Bank of Ireland, 8, 0, 22%
- TSB, 89, 3, 14%
- Danske, 6, 0, 13%
- Halifax, 70, 0, 11%
- Nationwide, 26, 4, 4%
- Virgin, 2, 0, 3%
- Coventry, 0, 0, 0%
- Cumberland, 0, 0, 0%
- M&S Bank, 0, 0, 0%
- Metro, 0, 36, 0%
And here are the percentages of bank branches lost through closures across the UK over the same period, according to Which?
- Wales, 43%
- North West, 40%
- South West, 39%
- Scotland, 38%
- Yorkshire and the Humber, 37%
- East Midlands, 34%
- West Midlands, 30%
- Northern Ireland, 29%
- South East, 29%
- East, 28%
- North East, 28%
- London, 25%
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