in a survey were from First Direct and HSBC, at £150 a month.
These charges applied to customers who were in the red for many days and made lots of payments from their accounts.
However, for the same overdraft, Barclays only charged £66.
The research also revealed that many banks are using expensive daily fees. Royal Bank of Scotland, for example, charged £6 a day, equivalent to 2190% APR on a £100 unauthorised overdraft.
First Direct and HSBC are also two of the banks currently offering incentives to encourage people to switch their current accounts.
First Direct has had a long-running offer of £100 to new current account holders, plus access to a regular savings account paying 8% for a year.
HSBC, as part of its "January sale", is offering to pay 6% interest on credit balances in current accounts opened before the end of January. This rate will be paid for 12 months on balances of up to £2500.
The New Year current accounts war began when Halifax started offering £100 to new customers who open a current account by February 19.
The offer, however, does not apply to Bank of Scotland, which is now part of a separate division within Lloyds Banking Group. Halifax's deal includes the offer of £100 immediately, upfront, when the customer applies in a branch. Santander responded by extending its own cash rewards scheme until February 29. It pays £100 to new customers who switch over their current accounts, and up to £300 to its existing mortgage customers who switch both their current accounts and their savings.
To take advantage of all these offers, customers have to agree to transfer a certain sum to their new account each month, ranging from £500 at HSBC to £1500 at First Direct. Santander is even more specific. In order to make sure people switch their main account, it not only requires them to credit their accounts with £1000 a month but there must be at least two standing orders or direct debits set up on the account they are switching from.
The Co-operative required the same commitment when it offered £200 to new customers last October – an offer so popular that it was withdrawn after only four days.
One catch with the Co-op's giveaway was a three-month wait for the promised £200, and only then if £800 a month had been paid in to the account each month.
As an extra incentive to customers to maintain their accounts for at least a year, Santander also offers 5% interest on credit balances of up to £2500 for the first 12 months.
Halifax's Reward Account pays £5 a month to current account holders, equivalent to credit interest of 5% on an average £1500 monthly balance. Unlike other offers, this one is ongoing – and it is available at Bank of Scotland.
To make it easier for people to switch, most banks nowadays also have special teams to help arrange the transfer.
David Black, banking specialist at financial research group Defaqto, explains the banks' apparent generosity.
He says "The main reason banks are prepared to give away these incentives is that if they have your main current account they will have a lot of details about your financial behaviour and will be able to cross-sell other services to you such as loans and personal pensions and recoup the money."
And, as Which? points out, they charge you hefty fees if you go overdrawn.
However, for anyone facing a temporary period when they may need to go into their overdraft, two of the banks presently battling it out for your custom may also be worth considering. Both HSBC and Santander are offering interest-free overdrafts for 12 months.
Barclays, which is not providing a cash incentive, also offers a 12-months-free overdraft, as well as being the cheapest for unauthorised overdrafts, according to Which?
Yet research by Halifax reveals that many people cannot be bothered to switch their current accounts. One-third of people still have the same accounts that they opened between the ages of 16-24.
Around the same proportion of those surveyed said that nothing would encourage them to switch bank accounts, although more than 37% said a better rate of interest on their current account would be an incentive to move. However if you are having service problems with your bank, make sure you are not jumping out of the frying pan into the fire.
The latest current account customer satisfaction survey showed Santander finished in last position.
First Direct came top of the table in terms of customer satisfaction, despite its high unauthorised overdraft charges. Other banks which scored well included Co-operative Bank and Nationwide.
Next year, Richard Branson will be entering the fray.
Following Virgin Money's takeover of Northern Rock, he announced in early January that it will start offering current accounts next year.
Initially there were plans for the accounts to be fee-charging but Branson has now said there will be "free" options as well. But consumers should not expect anything too radical. Virgin Money's investment funds actually have higher than average charges.