Almost a quarter of people feel they have been "ripped off" at some point in their lives by letting agents' charges, research from Shelter has found.
Around 23% of more than 5,000 people surveyed believed they had been landed with unfairly high fees for aspects of renting in England such as credit checks, renewing contracts and "administration".
More than half (52%) of people who felt they had been ripped off or knew someone who had said this was due to fees being "out of proportion" to the true cost of the work done.
The housing charity said it had found cases of renters being charged more than £150 for repeat credit checks every year, which Shelter said actually cost between £8 and £25 to perform.
It said some people were being charged £100 just to view a property and renters were being charged up to £540 in non-refundable "administration" fees.
Shelter also said it believes some landlords are being unfairly charged by letting agents. It said it also found some letting agents had been double-charging landlords and tenants for the same services.
People living in London, which has seen particularly fierce competition in the rental sector among tenants, were the most likely to believe they have been charged over the odds, with almost one in three (29%) of people saying this.
Kay Boycott, director of campaigns, policy and communications at Shelter, said: "It's scandalous that some letting agents are creaming off huge profits from the boom in private renting by charging both tenants and landlords fees that are totally out of proportion to the service they provide.
"With our investigation uncovering unexplained charges of over £500, we need to make sure that letting agent fees are reasonable. With costs like these, on top of the sky-high rents that families already face, it's no surprise that many dread the day they have to look for a new place to rent."
Rents have soared to new highs over the last year amid strong demand from tenants who have found themselves trapped in the rental sector.