Consumers have been warned to consider the cost of unauthorised borrowing by financial information provider Moneyfacts.

According to the research, while the average interest rate for going into an authorized overdraft is 12.1%, the rate for an unauthorised overdraft is nearly double this, at 23.3%.

If you go £100 into an unauthorised overdraft, you could end up paying as much as £160 in interest and fees if the debt isn’t cleared within 2 weeks, Moneyfacts revealed. Lloyds TSB’s Classic Plus account tops the table of the banks with the highest charges for a debt of £100 unpaid within 14 days, with a levy of £165.72. Most other banks, including Natwest’s Current Plus account and Abbey’s Current Account, charge in the region of £60, while Barclays charge around £30.

Lisa Taylor, analyst at Moneyfacts, said: “The bill for unauthorized debt could get much larger if you remain outside your limit for longer and if more items are presented and either paid or returned. This could really set your finances back during the first few months of 2008. If you are getting close to an agreed limit, don’t bury your head in the sand; the problem won’t go away. Visit your bank or pick up the phone and speak to an advisor – it’s got to be better than being hit with a further penalty that you can ill afford.”

She added: “Using an unauthorised credit facility will also damage your credit score with your current provider, making them less likely to be able to help you in the future should you require any further borrowing facilities.”