Barclays is facing criticism for opting out of part of the agreement banks have with Post Offices to allow customers to withdraw cash and deposit money.
The move is seen as a threat to efforts to provide services to communities that have seen branches and ATMs disappear.
Barclays will let its customers deposit money, but not withdraw cash from a Post Office counter using a debit card.
Consumer association Which? says it is a "shocking decision" which exposes the fragility of the UK's cash system.
The Payment Systems Regulator, which oversees the cash system, warned it was "concerned about the impact", while Rachel Reeves, chair of the Business, Energy and Industrial Strategy (BEIS) Committee, called the decision "unjustifiable", adding that Barclays needs to "think again".
The Post Office has unveiled a new agreement with 28 banks and building societies, covering the three years from January and allowing for postmasters and post mistresses to be paid more to take in and dispense cash.
Barclays is the only one to exclude cash withdrawals from its part of the agreement.
Instead it says it will launch a cashback scheme at small businesses in remote towns and areas where there is no branch or ATM alternative within 1km (0.6 miles).
Barclays has also promised to freeze last-in-town and remote branch closures for two years.
It says it is "committed" to its relationship with the Post Office because customers will still be able to deposit cash.
Natalie Ceeney, who wrote an Access to Cash Review this year, says Barclays customers make 1.2 million cash withdrawals from Post Offices every month.
She is calling on the bank to reverse its plan for the sake of people who still rely heavily on cash, saying: "The cash system that supports them must be cherished, not undermined."
The Payment Systems Regulator says the decision "reduces the number of places their customers can go to get cash", adding that it will be monitoring Barclays.
Ms Reeves said: "This decision comes at a time when, across the banking sector, High Street branches are closing and free cashpoints are under threat.
"It's essential that the future viability of the Post Office network is secured and unfortunately this decision from Barclays suggests they are forgetting their wider social responsibilities."
Which? has been concerned about a drift towards a cashless society. It is calling for legislation that guarantees consumers can continue to access and pay with cash for as long as it is needed.
The Post Office is clear that staff in its remote branches will get more money for handling banking transactions.
Barclays' line is that opting out and finding alternatives will be more efficient.
It says that 99% of its customers who use the Post Office are in an area where a branch or free-to-use cash machine is available as well.