The number of complaints made against letting agents has soared by 45% during the past year, the Property Ombudsman said today.
Christopher Hamer said disputes involving rental properties accounted for 49% of his workload during 2009, up from just 28% during the previous 12 months.
He expects the trend to continue during the coming year, leaving problems involving letting agents accounting for two-thirds of all the complaints he handles.
He said the rise was being driven by a combination of higher consumer awareness, more letting agents belonging to the redress scheme, and higher levels of activity in the letting market due to the housing market downturn.
But he added that many letting agents needed to "up the game" in the way they treated customers.
Mr Hamer called on whichever party wins the general election to introduce legislation to improve the protection for consumers and landlords who use letting agents.
He said: "To my mind this needs addressing swiftly. In the meantime, my message to any landlord or prospective tenant is to ensure that they use an agent who is a member of a recognised trade association or any agent who is a member of the Property Ombudsman scheme.
He added that it was an "alarming inconsistency" that letting agents were not forced to belong to a redress scheme under the Consumers, Estate Agents and Redress Act, although many letting agents have joined one voluntarily.
Despite the increase in disputes about letting agents, the Ombudsman's overall workload fell by 15% during the year, with the fall likely to have been driven by the drop in property transactions.
Overall, he resolved 562 disputes, down from 799 in 2008, with many of these relating to misunderstandings or a lack of clear communication between parties, rather than specific failings by agents.
At the same time, the average level of redress he awarded nearly halved to £339, down from £666 in 2008.
He said: "Every complaint is, of course, serious for the person who feels disadvantaged, but I can detect that the matters being presented to me are mostly less serious."
The Property Ombudsman offers a free service for consumers who are unhappy about the way they have been treated by an estate agent or letting agent who belongs to the scheme.
He can recommend compensation of up to £25,000, and his decision is binding on agents who belong to the scheme, although consumers can reject his findings and pursue the matter through the courts.
A Communities and Local Government spokesman said: "While the majority of letting agents and landlords act within the law and offer a good service, we are fully aware of the difficulties that some tenants face when things go wrong.
"That's why we are putting in place a wide range of measures, including legislation that will regulate letting and managing agents and establish a national landlords' register, that will empower private tenants and support a strong, well-functioning private rented sector.
"At the same time, we are putting in place a dedicated system of support for tenants through a national helpline and website for help and advice."