A High Court test case starts on Monday that could bring a fundamental change to High Street banking in the UK.
The outcome may decide the level of charges that banks are able to levy on millions of customers who go overdrawn without permission every year.
The Office of Fair Trading (OFT) will challenge seven leading banks and the Nationwide building society.
The regulator claims the banks’ overdraft charges are unfair, but the banks say they are entirely legal.
“We have always believed that what we are doing is correct and legal,” said a spokeswoman for the British Bankers’ Association (BBA).
“We are confident and think the hearing is an important opportunity to bring some clarity to the legal position,” she added.