Banks should end all unauthorised overdraft charges because they are trapping people in persistent debt, the financial charity StepChange has said.
The organisation also wants banks and regulators to do more to identify people caught up in a "vicious cycle of borrowing".
Even where customers have admitted they are in trouble, it said banks often fail to help.
In 2016 some 2.1m people used their overdraft every month of the year.
The UK's biggest bank, Lloyds, has already abolished charges for unplanned overdrafts.
Customers of Barclays cannot get an unauthorised overdraft, but they can apply for "emergency lending".
However others, including Santander, Royal Bank of Scotland (RBS) and NatWest still offer unplanned borrowing.
Santander charges up to £95 for each month a customer is overdrawn. RBS, NatWest and HSBC have a cap of £80 a month.
As part of its study into high cost credit the Financial Conduct Authority (FCA) said it was considering a ban on charges for unplanned overdrafts – but it is not due to report until 2018.
Half of StepChange's clients have overdrafts and each owe an average of £1,722.
They often use their overdrafts to pay off household bills, so end up in a cycle of debt.
Furthermore banks insist on giving customers large overdrafts, even though they may not be able to afford the repayments, the charity claims.
One of its clients was offered an overdraft of £2,250, even though they were working part-time and on benefits.
It says such banks are guilty of "unaffordable lending".
"Lenders and regulators must take action to need to ensure that overdraft lending is affordable, that borrowers in financial difficulty get the right support and that we break the cycle of persistent overdraft debt," said Peter Tatton, head of policy at StepChange.
"Fundamental reform is needed. There has been positive action from some banks to make charging structures clearer, and to abolish unarranged overdraft charges."
Clients also complained that even when they told their bank about their financial difficulties, little was done to help them.