Legal proceedings seeking a test case on the fairness of bank charges begins in the high court today.
The Office of Fair Trading (OFT) is seeking a declaration on the EU law relating to banks' use of charges for unauthorised overdraft usage.
It says the penalty charges are not justified under regulations governing the fairness of terms in consumer contracts, a claim rejected by the banks involved in the test case.
Abbey National, Barclays, HBOS, HSBC and Lloyds TSB are among the participating banks, which provide current accounts for 90 per cent of the UK market.
If the test case result backs the papers filed today it will establish a "legal principle clearly in the court", the OFT said in a statement.
"Tens of thousands of complaints that these charges are unfair have been received by the county courts and the Financial Ombudsman Service," it added.
"The OFT considers that a quick determination of this point of principle will assist in securing a clear and orderly resolution of the fairness of these charges."
In March the trading watchdog launched its formal investigation into unauthorised overdraft charges, following its announcement that it shared the public concern with the "level and incidence" of current charges.
The British Bankers' Association has joined the OFT in seeking clarification, but chief executive Angela Knight told the Today programme it was "a technicality as to which is on which side".
"Of course because matters do take a considerable time and because of the detail and complexity of the issue, we felt that it was fairer to the customers for us to get some clarity and that is why we, in conjunction with the OFT, are approaching the courts," she said.
"It will take a little while but we feel that this is the much fairer way forward."