People living in rural areas are having to travel further to find somewhere to withdraw and deposit cash free of charge, says the City regulator.
Almost every urban resident has access to a bank, building society, post office or ATM within 2km of home, the Financial Conduct Authority (FCA) said.
This drops to three-quarters of the UK rural population, its research found.
While longer distances are expected for rural residents, campaigners fear vulnerable people face difficulties.
In rural areas of the UK, fewer than half (45%) of residents have a bank branch within 5km of their home.
The FCA said it was considering stronger requirements of the sector to ensure five million people who rely on notes and coins are able to have access to it.
"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," said Sheldon Mills, executive director at the FCA.
"We will also review over the coming months how we can strengthen our guidance to help protect reasonable access to cash and banking services."
Withdrawals from cash machines in the UK fell by £37bn during the first 12 months of the Covid pandemic.
The FCA research found that a greater proportion of consumers were finding it difficult to cope with fewer retailers accepting cash. However, it found that eight out of 10 small and medium-sized businesses said they were "very likely" to accept cash over the next five years.
Uptake of digital money services has increased, but there are concerns that the technology does not suit everyone, with some of the most vulnerable people in society still relying on cash.
Right to cash
Charity Age UK said that people required the same guarantee of access to cash as they did for running water, electricity and the post.
Closures of cash machines and bank branches, as well as risks to the continued economic viability of network of producing, transporting and sorting cash, have prompted government and regulators to take action.
The Treasury has already thrown its weight behind the use of local shops as an alternative to ATMs. It is keen on the idea of shops offering cashback to customers, even if they have not made a purchase.
The government has also published plans to ensure consumers and businesses have a legal right to withdraw and deposit cash within "a reasonable distance" of their home or premises.