A banking watchdog has been urged to step in amid concerns that consumers could be left struggling to access free-to-use ATMs.
Consumer group Which? raised concerns there could be mass closures of cash machines due to a row over how the network is funded.
Last year 2.7 million people in the UK were reliant almost entirely on cash, and they could be badly hit, the group has warned.
It has written to the Payment Systems Regulator (PSR), asking it to conduct an urgent market review to ensure consumers' rights are protected.
The PSR has also said it is monitoring the situation closely - and it will take regulatory action as necessary if it considers that any decision made by the UK's ATM network Link is not in the interests of consumers.
The call comes amid a simmering row over planned changes to interchange fees charged between banks to use each others ATMs which are used to fund the free system.
Banking group Link, which runs some 70,000 ATMs across the country, has proposed to gradually cut the interchange fees over the next four years from around 25p to 20p per withdrawal.
It has recently launched a consultation into the planned reductions.
Link has said the plans will help to retain an extensive network of free ATMs for consumers.
But trade body the ATM Industry Association has warned that "ATM deserts" could be created.
In its letter to the PSR, Which? said it has "significant concerns" about the impact for consumers and does not believe proposals should be allowed to proceed without sufficient scrutiny from the regulator.
It said a wider review should consider potential alternative options for accessing cash and include a full examination of the ATM market.
A spokesman for the PSR said they had made it clear to link that they must consider users' needs when deciding whether to reduce the fees for banks.
He added: "If we consider that any decision Link makes is not in the interests of consumers, and is therefore inconsistent with our statutory objectives, we will take regulatory action as necessary."