Lenders are banking on a reversal of fortune in the House of Lords as they seek to overturn a ruling that puts them in line to refund £1bn to customers.
It is the last chance for seven major banks and a building society to win their legal row over whether bank charges for unauthorised overdrafts are subject to regulation by the Office of Fair Trading.
They are asking the Law Lords to overturn High Court and Court of Appeal decisions that the charges come under "unfair contract"
rules and are therefore open to scrutiny by the OFT.
Tens of thousands of customers whose refund claims have been frozen while the test case goes through the courts will be eagerly awaiting the final ruling.
But it will also pave the way for a further hearing to decide whether the charges are fair and, if not, what a fair charge would be.
Customers who go into unauthorised overdraft or breach their agreed limit can be charged as much as £35 for a single bounced payment.
Campaigners claim the actual cost to the banks could be as little as £2.50.
If the banks ultimately lose the test case, it could cost them £2.6bn a year in lost revenue and lead to their having to make refunds of up to £1bn.
Before refund claims were frozen, some banks had already paid out more than £559m to customers who complained about "rip-off" overdraft charges.
Industry leaders have warned that defeat in the House of Lords could lead to the end of free banking in the UK, with consumers having to pay a monthly fee or a fee for every transaction they carry out.
But many of the high street banks have already changed the structure of the fees they charge people who go into the red, with or without permission.
The test case to decide the legal issues thrown up by the dispute was brought jointly by the OFT and Abbey; Barclays; Clydesdale; Halifax Bank of Scotland; Lloyds TSB; HSBC, Royal Bank of Scotland and Nationwide Building Society.
The final appeal hearing, before five Law Lords, is set for three days and judgment will be reserved to a later date.