Overdraft charges: '8m more customers could claim their money back'

New figures from the group show that, of around 12m people who have been hit with bank charges since July 2001, over three in five (63pc) have not asked for their money back.

With the Supreme Court's ruling on the test case due on November 25, the banks could face a deluge of new complaints should the Office of Fair Trading (OFT) win. This is in addition to the estimated 1m or more claims that are currently on hold.

Around four in 10 claims (42pc) waiting to be resolved are for amounts of £500 or more. Four in 10 people waiting for a claim resolution (41pc) saying that they would use any refunded money to pay off bills or debts and around two in 10 saying they would spend it on shopping (20pc) or a holiday (18pc).

Phil Jones of Which? said: "If the Supreme Court rules against them, the banks could find themselves staring down the barrel at millions of fresh claims. This is in addition to the huge backlog that's already built up since the waiver was put in place.

"With so much money at stake, the banks may well try to drag the process out even longer, so we'd warn people not to make any plans for their refunded cash just yet."

Which? has launched a new banking campaign – Britain Needs Better Banks – and anyone who has been treated shoddily by their bank can visit www.bnbb.org to share their experience or for information about how to make an official complaint.

The British Bankers' Association said: "It is too early to speculate on the outcome and consequences of Wednesday's Supreme Court judgment. But the estimates [from Which?] seem high and do not reflect the vast majority of current account customers who do not incur charges as they have taken the simple step of making arrangements for adequate overdraft facilities with their bank in advance of spending more than is in their account.

"Wednesday's Supreme Court ruling is not about the fairness of bank charges – that is subject to a separate and ongoing Office of Fair Trading investigation which is yet to conclude. The case will only look at whether a piece of EU legislation can be applied to current accounts."

Which? questioned 4,161 adults aged 16 or over in September 2009.