Have you ever been caught in the cash machine trap?

You need a tenner quickly and you're scrambling around looking for a machine. You find one, you put in your pin and a message appears on screen: "You will be charged £1.85 for this transaction. Do you wish to proceed?" No, you do not wish to proceed and be charged nearly 20% to take out your own money. So you look for another cash machine. Problem is: it charges too. In some areas, it can be hard to find a machine that does not charge.

It is worst in the most deprived communities. Recently, leaked figures from Link, the body that is responsible for running cash machines in the UK, revealed 269 low-income areas lack a free machine within a 1km radius - which according to one estimate leaves 300,000 poor people without easy access to a free machine. I've seen it for myself in some of the old mining communities in Ayrshire, where I live.

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It is an obviously and grotesquely unfair situation- for the good reason that the poorest are most likely to live on ready cash and are least able to afford the charges - but it has happened largely because of the move from banking-by-branch to banking-by-computer. That's fine for most people, but it has also caused the closure of branches, and cash machines, which in turn has allowed private companies to move in and install machines that charge.

Campaigners willing to raise this issue have been few and far between, but one who has is the Labour MP Frank Field and he was doing it again yesterday. Using the word "scam", he called on the Government to tackle ATM charging and other ways in which the poor pay more than the rich for services - the so-called poverty premium.

I agree with Mr Field, but the solution is not, as he suggests, more government regulation - that is rarely the solution to anything - it is a reform of one of the fundamental ideas of British banking: the principle of "free banking". This principle - that current accounts in credit should cost you nothing - has always been popular in the UK, but it seems extraordinary to me that in all the recent talk of pay and bonuses, state ownership, regulation, the division between retail and investment banking and the proper scrutiny of appointments (ie no more Rev Flowers), no one has consistently or profoundly questioned the principle of free banking. And yet it is the one idea - much more so than bankers' bonuses - that specifically punishes the poor.

The problem is this: if you are well-off and have an account that is always in credit, you are never charged for the service you use: the banks, the cash machines, the staff, all of it comes for nothing. But the banks still need to pay for it somehow and so the charges are heaped on those who get into trouble - those who go into overdraft for example or are forced to take out a loan. In other words, it's the same old story: those who can least afford it have to pay the most.

However, there is a fairer and more transparent way of doing things which is to put the costs up front and charge for accounts in credit. Ideally, this would be on a sliding scale so those with a small amount in their accounts would pay a modest fee and those with better balances would pay more. Not only would this distribute fees fairly, it would allow banks to be upfront about what it costs to run their services rather than hiding the costs in the shadows. It is partly because banks have not been allowed to be upfront - and are forced to maintain the illusion of free banking - that they have had to meet their costs elsewhere by closing branches, which in turn has allowed private firms to move in and install cash machines that charge customers for withdrawals. Allowing banks to charge upfront could help protect the cash machine network and keep the private providers at bay.

In recent years, a few people have been willing to question the free banking model - Mike Dailly of the Govan Law Centre, for instance, and Ross McEwan, the chief executive of RBS - but they are shouted down and that's a pity because it's not just bankers who need to change their habits - it is those who use banks too. Specifically, customers in credit need to be reminded their "free" banking is paid for elsewhere - by the person forced into overdraft, the person hit with charges when a cheque bounces, the person who has to pay £1.85 to take out £10 of their own money.