A survey of National Residential Landlords Association (NRLA) members found that 66% now use a letting agent in some capacity.
According to the NRLA, that is the highest proportion recorded for at least two years.
The research, conducted by Pegasus Insight, comes as landlords prepare for further changes under the Renters’ Rights Act.
The NRLA says the growing reliance on agents strengthens the case for minimum education and training standards.
It has also warned about agents who lack sufficient knowledge of their legal and regulatory responsibilities.
The association pointed to official figures showing differences in tenant satisfaction with repairs.
Some 59% of tenants in fully agent-managed properties said they were satisfied with the repairs service. That compares with 73% of tenants whose properties are managed directly by their landlord.
The NRLA is calling on the government to accelerate plans for minimum education and training requirements for agency staff.
Ministers have so far committed to consulting on the issue next year.
Ben Beadle, chief executive of the NRLA, said: “Landlords are ultimately responsible for ensuring the properties they rent meet all required standards. However, with two thirds of rental properties now entrusted to some degree to the care of an agent, it is vital that they and their tenants can have confidence in the services that these letting agents provide.
“As the rental market becomes more complex, reliance on letting agents is only likely to grow further. Reforms to the agency sector should therefore go hand in hand with the wider changes to the rental market.
“Whilst most agents provide a good service, too many do not. Minimum training standards would help ensure those working in the sector understand their responsibilities and can provide a consistently professional service.
“Pledging yet another consultation next year is simply not good enough. Tenants, landlords and reputable letting agents deserve action now.”
