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% 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% 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% 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% branch reduction, while Carmarthen East and Dinefwr lost four-fifths (80%) of the network.
Wales has seen 43% of its bank branches shut, while for Scotland the percentage is 38%, and in Northern Ireland it is 29%, 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."A spokesman for UK Finance said: "Bank branches play an important role in the life of local communities and decisions to close them are never taken lightly.
"Research shows that consumers are increasingly choosing new ways to help them with their banking, using technology to check balances and make payments - or even speak to their bank 24/7.
"But technology is not for everyone and maintaining access to cash is vital to ensure no-one is left behind."
He said the industry continues to work closely with the Joint Authorities Cash Strategy Group, Government, Bank of England and regulators.
The spokesman said: "An update on this work is expected in the coming weeks."
Caroline Abrahams, charity director at Age UK, said: "Many older people face challenges when it comes to managing their money and a rapid move towards online banking in recent years has undoubtedly caused significant problems for many customers, particularly those with visual impairments and dexterity problems.
"These problems are exacerbated when branch closures coincide with poor public transport locally, substandard internet service and mobile black spots, making both digital and analogue alternatives difficult for customers to access."
Federation of Small Businesses national chairman Mike Cherry said: "There are still actions that we as small business owners have to sign off in a branch, in person."
Mr Cherry said high streets could "end up damaged beyond repair" by rapid bank branch closures.
An HSBC UK spokesman said the bank continues to invest in its network, including refurbishing many branches.
He continued: "We have closed four branches since December 2017 and never take these decisions lightly."
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%