It wants Barclays, HSBC, Lloyds TSB and NatWest (now owned by Royal Bank of Scotland) to increase the rate of interest from 0.1% to 2% on customers' balances when they are in credit.

Of course we are not ripping off customers

Lloyds TSB

And it said overdraft rates should be cut to 10%.

Although the best overdraft rates are less than 9%, the so-called 'big four' banks charge up to twice this amount.

Which? calculated that if the 70% of customers who bank with the big four switched to one of the best accounts around, between them they could save up to �500m a year.

Easy

A spokeswoman for Lloyds TSB said: "Of course we are not ripping off customers.

Customers have the power to make the banks take notice

Helen Parker, Editor Which?

"They choose to bank with us and if they are not happy, they move."

She said the bank tried to make it as easy as possible for customers to move their accounts to other banks.

And she said people were not only concerned with the level of interest on current accounts and overdrafts.

Customer power

"There are quite a lot of differences between current accounts, for instance we have got the ability for customers to pay bills over the counter and they have access to 2,100 branches.

Helen Parker, the editor of Which? said: "Because most people don't switch, the big banks have no reason to change.

"But customers have the power to make the banks take notice."

The criticism from Which? came as the heads of the four big banks were giving evidence to MPs about their treatment of business and personal customers.

The government has already imposed price controls after accusing the banks of overcharging small businesses and making excess profits.

The Treasury select committee, which is hearing the evidence, could make further recommendations to the government.