Sefton Central MP Bill Esterson has described as "appalling" a decision by Barclays Bank to stop allowing its customers to withdraw money at the Post Office.
The bank, whose Maghull branch is one of 481 which have closed since 2015, had previously reassured customers that they would still have access to cash through the Post Office even after their local Barclays had closed.
Mr Esterson backed a letter signed by a group of 124 MPs to Barclays chief executive Jes Staley to ask the bank to reconsider its decision to stop customers withdrawing cash at Post Office counters.
Mr Esterson said the decision let down the loyal Barclays customers in the constituency. He said: "The decision by Barclays to stop its customers from withdrawing cash from the Post Office is appalling and goes back on assurances the bank made to customers that they would be able to use the Post Office after local branches closed down.
"The decision is said to be saving the bank £7m a year which is a drop in the ocean for an organisation like Barclays which made a profit of £3.5bn last year.
"I support my colleagues' calls on the bank to reverse this decision which will create a crisis for so many, especially the elderly and most vulnerable, who either don't have access to cash machines or don't feel confident or safe using them."
The Access to Cash Review, which was published in April, found that 17 per cent of the UK population – about 8 million people – would struggle to function without access to cash.
Mr Esterson added: "It is clear this decision will hit the poorest hardest which is completely unacceptable for a bank like Barclays, which should be assessing the impact on communities of the decisions they make. This should not be just about the balance sheet and what might save money. The impact on customers should be paramount."
The MP said Labour's plans for a "Post Bank" would put communities first. Earlier this year the Shadow Chancellor John McDonnell announced that Labour in government would set up a Post Bank based in the Post Office network.
Mr Esterson said: "A publicly owned Post Bank would give every community easy access to face-to-face trusted and affordable banking. It would be by far the largest branch network of all UK banks.
"Labour would create thousands of Post Bank branches in existing post offices to help support. Our plan would all help to revitalise our high streets and bring footfall into shopping areas as well as helping the 8 million people who rely on cash and thousands of businesses which handle cash each day."
Research by Which? has revealed that the UK lost almost two thirds of its bank and building society branches over the past 30 years. A fifth of the population of the UK lives two miles or more from a bank. In the last five years 40 per cent of the Crown Post Office Network closed, which equates to 150 branches.
In Merseyside, half of all bank branches have closed - 160 - since 2010.