A trade minister has told customers of two major banks who refuse to allow them access to their cash via post offices to consider closing their accounts.

Liberal Democrat Ed Davey sharply criticised HSBC and Santander for being the only remaining major institutions not to sign up to using the network.

The Government is keen to widen the practice as part of efforts to revitalise post offices after deciding against the creation of a Post Office Bank.

A consumer group estimates that if high street banks allowed their customers full access to personal bank accounts in post office branches, customers could soar from four million to 18 million.

In his speech to the Liberal Democrat party conference in Birmingham, he announced that Nat West customers would this week become the latest to enjoy some post office access.

“In fact, just two banks are holding out – HSBC and Santander,” he told activists.

“Frankly, that’s really disappointing. So, I hope you will join me in calling on them to think again. And if they don’t respond, maybe we need another campaign. To tell these banks they have a responsibility to their customers and local communities.

“And if they still don’t listen. Well, I hope you will think about switching your account to another bank.”

Even banks that do offer access through the post office often do not provide the full range of transactions such as making a cash or cheque deposit.

Consumer Focus said last month that there was “real appetite” for such access.