Customers who exceed their overdraft limit without permission from their bank or building society pay around 25.4 per cent in interest on the excess, MoneyExpert has found.
Unauthorised overdraft usage costs customers around twice as much as authorised usage, according to the study, which suggests that customers who anticipate the need to extend their overdraft in advance can cut rates to 12.6 per cent.
"Make sure your authorised limit is big enough and if it isn't tell your bank before you go over the limit," advised MoneyExpert's chief executive, Sean Gardner.
Mr Gardner described unauthorised overdraft rates as "punitive", but backed banks and building societies which charge customers penalties.
Earlier this week, a study from price comparison website uSwitch.com found that banks collected an extra £624 million in 2006 by increasing authorised overdraft rates and credit card APRs.
In December the BBC's Money Programme calculated that it costs only £4.50 for banks to handle current account defaults – but they charge as much as £30 in fines.
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