High street banks are hitting customers with overdrafts fees eight times higher than the cost of a payday loan, research has found.
Consumer watchdog Which? found bank customers who borrowed £100 for 30 days through an unarranged overdraft could pay up to £180 in charges.
The same loan from a payday loan firm over a similar period has been capped by regulators at £24.
Which? said bank charges mount up because they apply over each monthly billing period when a customer is overdrawn, not the number of days over which the money is borrowed.
That means customers could face paying two sets of charges if the 30 days of being overdrawn stretches across two separate months.
The watchdog found that Natwest charges up to £180 if customers borrowed the money across two monthly billing periods. Customers of Lloyds or Santander could be charged up to £160.
About a fifth of current account holders have unarranged overdrafts, which generated £1.2bn of revenue for banks in 2014, according to the Competitions and Markets Authority (CMA).
Which? director of campaigns and communications Vickie Sheriff said: "It's not right that people with a financial shortfall can be charged so much more by the big high street banks than they would by a payday loan company – especially if the money is borrowed over two monthly charging periods.
"If banks can continue to set their own charges, then consumers will continue to be hit by exorbitant fees."
Regulator the Financial Conduct Authority (FCA) is in the middle of a review on whether to cap unarranged overdraft charges, and plans to publish its findings by the summer.
Sheriff called on the FCA to "cap these high charges and ensure consumers cannot be charged more for unarranged overdrafts than arranged overdraft".
Natwest, Lloyds and Santander all called on their customers to contact their branches if they feel they are about to go into an unplanned overdraft.