Around a quarter of people living in rural areas do not have free access to cash within 2km (1.2 miles), according to figures published by regulators.
The Financial Conduct Authority (FCA) said that, while most people have reasonable access to cash, rural access is lower than urban access.
Some 99.7% of people in urban areas have access to a free source of cash within 2km (1.2 miles).
But this falls to only around three-quarters (76.6%) of the UK rural population.
Looking at a 1km radius, only around half (around 53%) of people in rural areas have free access to cash within this distance, compared with 95.4% of the UK urban population.
Overall, 87.4% of people have free access to cash within 1km.
The FCA published the assessment of the UK’s cash infrastructure jointly with the Payment Systems Regulator (PSR).
Cash access is provided through a combination of banks, building societies, Post Office branches and ATMs.
The FCA said that, when access is removed, there is a risk of harm for those who depend on cash often because of a characteristic of vulnerability such as ill-health, a life event, low financial resilience, or low financial or digital capability.
The FCA, PSR and other bodies are working to prevent harm from happening.
The regulators estimate 95.4% of the UK population generally are within 2km (1.2 miles) of a free cash access point and 99.7% are within 5km (3.1 miles).
The rural population travelling further to withdraw and deposit cash can reflect higher distances to amenities generally, such as shops, facilities and public services, researchers said.
Some 98.2% of the UK rural population are within a free source of cash within a 5km (3.1-mile) distance.
Overall, data suggests that 99% of the UK rural population have access to a free source of cash within 5.7km (3.5 miles), the FCA said.
Cash use nosedived during the coronavirus pandemic, with many retailers discouraging shoppers from using it.
Coins and banknotes were used for 17% of all payments in the UK last year, while 27% of payments were contactless, according to figures recently released by trade association UK Finance.
According to the FCA’s research, one in six adults said fewer businesses accepting cash during the pandemic has caused them difficulties.
Nearly 80% of businesses said they are very likely to accept cash over the next five years.
At the end of the first quarter of 2021, the vast majority of bank and building society branches were operating on reduced hours compared with February 2020 – before the coronavirus lockdowns started.
The UK Government has said it will legislate to protect future access to cash.
In September 2020, the FCA published guidance on how it expects firms to approach branch and ATM closures or conversions to make sure their customers are treated fairly.
The PSR recently published a review to help ensure ATM network Link does all it can to make free-to-use cash machines available for UK consumers.
And the Treasury has launched a consultation on establishing geographic requirements for the provision of cash withdrawal and deposit facilities.
Sheldon Mills, executive director, consumers and competition at the FCA, said: “Around five million adults say they still rely on cash, and we know that, where access is removed, it can affect the most vulnerable in society.
“This is why we have intervened in the past to provide banks with guidance on what to consider when closing branches.”
He added: “There are still pockets of consumers, some displaying characteristics of vulnerability, who do not have sufficient access to cash.
“We expect firms to help protect access to cash and wider banking services in ways that meet consumers’ needs, and we continue to engage with firms closing their branches, to ensure that they treat their customers fairly.
“We will also review over the coming months how we can strengthen our guidance to help protect reasonable access to cash and banking services.”
Sarah Coles, personal finance analyst at Hargreaves Lansdown, said: “Could anyone with mobility issues cover a 4km round trip each time they need money – especially if they don’t have transport?
“When the distance is narrowed down to 1km, the numbers look far less promising, with only around half of people in rural areas this near to their money.”