Calls are being made for rental sector tenants to be given greater legal protection, which would see rogue lettings agents put out of business in the worst cases.
Which? said that lettings agents should be governed by the same laws as estate agents and the Office of Fair Trading (OFT) should have the power to ban lettings agents who break the rules. There are 4.7 million UK households in the private rental sector and two-thirds of private tenancies involve an agent.
But Which? said these households face a "gamble" as to whether they get a good or bad agent. Four out of 10 tenants it surveyed said they thought the up-front fees being charged are unfair and the group said it had seen evidence of some lettings agents using "aggressive sales tactics".
Some tenants are also being charged up to £90 to renew a tenancy and up to £120 to "check out" of a property, the consumer group said.
Which? said that lettings agents should include their fees in the headline price and made clear at the point of sale, in adverts and on websites. Agents should also provide full details of the terms and conditions of the agreement before any upfront fees are paid, the body said.
It said that two-thirds (62%) of tenants and nearly half (45%) of landlords it spoke to said they did not know whether their lettings agent belonged to a professional body. More than 1,500 people took part in the research for Which?.
Which? executive director Richard Lloyd said: "People searching for a rented home through a lettings agent are too often hit by unexpected and unfair fees or unacceptably bad service. With the private rented sector now the only option for millions of people, it is vital that more is done to protect both tenants and landlords from rogue lettings agents."
Demand in the rental sector has rocketed as people have been unable to buy their own home, either because they cannot raise the deposit needed or meet lenders' toughened borrowing criteria in the difficult economy, which has pushed up the cost of renting.
Housing Minister Mark Prisk said: "I'm appalled that some agents are abusing their position and giving the whole industry a bad name. We are determined that all tenants receive a good service, but we want to avoid excessive red tape that would push up the cost of rents and reduce choice for tenants. That's why we have strongly backed industry-led schemes such as SafeAgent."
Mr Prisk added: "Letting agents are already subject to consumer protection legislation, so any tenants who have suffered from poor practice should report this to their local trading standards officer or the Office of Fair Trading."