A Government minister has urged customers to consider boycotting two major banks who have refused to allow them access to their cash via post offices.

Liberal Democrat Ed Davey, the Post Office Minister, 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 branches, customers could soar from four million to 18 million.

Mr Davey also confirmed the Government is to press ahead with plans to transform the network into a John Lewis-style partnership.

It could mean sub postmasters, customers and communities are given a say in how the Post Office arm of the network is run, including the appointment of directors.

Ministers said the Post Office could be on a "clear path" to mutualisation by the end of this parliament as it launched a consultation of its proposals.

Post offices play a vital role in sustaining many Westcountry communities, particularly in rural areas. The last government oversaw the closure of 140 branches in Devon and Cornwall as part of the nationwide programme to axe loss-making operations.

In his speech to the Liberal Democrat party conference in Birmingham, Mr Davey 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 today, 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."

HSBC said a survey had shown its customers do not want to access banking through the Post Office.

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.

A Consumer Focus study of more than 2,000 adults showed that half of those who accessed their current account at a post office rather than a bank found it easier, while a third complained that their bank had no branch in their area.

Sarah Lee, head of policy for the Countryside Alliance, said the benefits of mutualisation would only be realised after the next general election, which will be too late for many.

She added: "The Government tell us that they would like to see the Post Office as the front office of Government services, they say they value the Post Office network, they say that they want to put the loss-making network on a sound financial footing, so it is disappointing that today's proposals do not go far enough or come soon enough to safeguard the very network they tell us they cherish."