About 2.5m people used an unauthorised overdraft in the last year, with over two thirds saying charges on them are too high or unfair.
The poll for consumer magazine Which? found over a third of people (36%) were surprised at how high their charges were, with 68% saying they were too high.
Which? said 25,000 people had now signed up to a campaign called 'Stop Sneaky Fees and Charges' aimed at misleading or overly high fees.